Gem-A at International Jewellery London 2019

International Jewellery London is just around the corner and all of us at Gem-A are hugely excited for what promises to be a scintillating showcase of jewellery and gemstones!

If you will be attending IJL 2019, we would like to invite you to join us at Booth F21, where you can meet our team, find out about our courses and try out some practical gemmology with our tutors!

While in recent years we were located in the upstairs section of the show, this year we are positioned at the front of the show (on the right hand side as you walk through the door) in a learning hub alongside the NAJ and The Goldsmiths’ Centre.

Guide to International Jewellery London 2019

Sunday 1 September 2019

Presentation: Gem Empathy Award Winner 2019

Location: Gem-A Booth, F21

The entries have been judged and one lucky IJL exhibitor/visitor will be announced the winner of the Gem Empathy Award 2019 and take home the 12.9 carat precious coral cabochon sourced and cut by Enzo Liverino 1894.

Tuesday 3 September 2019

Seminar: Precious Coral & Sustainability

Location: Catwalk Theatre

Read more: Misunderstandings about Precious Coral

Rui Galopim De Carvalho FGA DGA, Vice-President of the CIBJO Coral Commission, dives into the underwater world of precious coral and addresses some of the misunderstandings around its use in the jewellery industry.

Daily at the Gem-A Booth

Across all three days at IJL 2019 we shall be running in-booth gemmology training sessions at our Practical Gemmology Lab! These fun, 20-minute sessions run by our brilliant team of tutors will help you get to grips with the basics of gemstone identification:

10:30 am:  The Diamond Detective

Learn the basics of separating diamonds from its simulants and synthetics.

12:30 pm: A Gemmologist’s Best Friends

Get the most out of your loupe, lights and the microscope.

2:30 pm: Become a UV Jedi

Master the use of UV to distinguish between similar-looking stones.

4:30 pm: Spot the Difference

Separate blue stones with the use of the dichroscope and Chelsea colour filter.

 

Are you a gem detective? Drop by the booth at any time and test your knowledge! Cast your eye over eight different blue stones and try to identify the gems! If you are new to gemmology you can get some help from our gemmology tutors or use the clues provided.

Exclusive Offers for IJL Visitors

We are pleased to invite you to take advantage of special discounts and offers, exclusively reserved for exhibitors and visitors at IJL, including 15% off Gem-A one-day workshops and our GemBasics course when you book at the show!

What’s more, we are offering two fantastic instrument bundles at a significantly reduced rate – an offer only available at the show and while stocks last! IJL visitors and exhibitors can choose from:

Offer 1: The Filters Bundle

Includes: Chelsea colour filter, dichroscope, flat light and Maglite all for £75 inc. VAT, saving £23.40.

 

Offer 2: The Power Couple

Includes: A desktop microscope plus PhotoAtlas Vol. 3 for £1650 inc. VAT plus free UK shipping, saving £220.

 

We cannot wait to see some of the fascinating presentations lined up for the Inspire Seminar Programme, fresh designs from emerging talent and of course the amazing array of gems and jewellery on display. If you haven’t already, head to the IJL website to register! We look forward to seeing you there!

We have some fantastic guest speaker events planned for October, including a workshop on Organics with Gem-A President, Maggie Campbell Pedersen FGA ABIPP and a workshop on The World of Antique Jewellery with jewellery historian and valuer, John Benjamin FGA DGA FIRV. To book tickets, head to the workshops webpage

Additional Info

Read more...

A First Time Visit to International Jewellery London 2019

Gem-A’s communications assistant, Olivia Gillespie, reflects on her first experience at one of the UK’s biggest jewellery trade shows, International Jewellery London.

Having never visited International Jewellery London before, it was difficult to know what to expect! Upon walking through the front door of Olympia London on Sunday, the opening day of the show, I was quite taken aback by the enormity of the venue and the masses of visitors and exhibitors already networking and eyeing the many dazzling jewels on display.

I was also quite surprised by the variety of exhibitors, ranging from jewellers and gem wholesalers, to watch dealers and educational institutions, and by how far some had travelled to be at IJL, with some being based as far afield as Denmark, Hong Kong and Canada. There was a veritable feast of jewels and gems to behold at IJL, but a few remarkable exhibits and events made an especially sparkling impression on me…

 The amethyst 'Victoriana' necklace by Fei Liu Fine Jewellery.

International Jewellery London: Fine Jewellery Design

IJL 2019 had numerous exhibitors showing fine jewellery but I was particularly dazzled by the beautifully elegant statement jewels from Fei Liu Fine Jewellery. Fei Liu was showing his new collection, Victoriana, which is inspired by popular gemstones and stone cuts from the Victorian period, most notably the octagon cut, which featured in an exquisite suite of amethyst jewellery on display.

Two stunning statement rings by Fei Liu Fine Jewellery. Ring 1: 18 carat white gold ring set with a 17.35 carat green tourmaline and surrounded by 1.92 carats of tsavorites and 2.19 carats of baguette-cut diamonds. Ring 2:18 carat rose gold ring with a 9.35 carat garnet, 1.68 carats of diamonds and 4 carats of rutilated quartz.

I was utterly transfixed by two glorious rings on the brand's stand. The first is an 18 carat white gold ring set with a 17.35 carat green tourmaline and surrounded by tsavorites and baguette-cut diamonds, while the second is an 18 carat rose gold ring featuring a 9.35 carat garnet centre stone, flanked by 4 carats of rutilated quartz and bordered by diamonds.

Unusual Gemstones

As a member of the Gem-A family, I always get excited to see unusual coloured gemstones. I was very happy to get the chance to peruse the collection of gemstones exhibited by Ntinga, a London-based fine jewellery brand, which included a brilliant array of interestingly gems, including tanzanite, watermelon tourmaline, ametrine, iolite, indicolite and a fiery orange sunstone.

Sunstone from Ntinga

The IJL Catwalk

I made sure to pay a visit to the first catwalk of the day which showcased stunning jewels for women and men from the show’s exhibitors. It was great to see British gemstones as well as British jewellers on display, as one of the models sported CW Sellors’ silver and jet octopus ring and octopus pendant, bracelet and necklace on the catwalk. The huge Whitby Jet jewels on display at CW Sellors’ stand also caught my eye, especially the somewhat spooky spider necklaces!

 Whitby Jet jewellery from CW Sellors

Gem-A's Gem Empathy Award

The first day of IJL also saw the presentation of the Gem Empathy Award, an annual jewellery design competition run by Gem-A and hosted at IJL. Entrants were asked to conjure a design featuring this year’s prize stone, a 12.9 carat precious coral cabochon sourced from Enzo Liverino 1894.

Read more: Breaking Down the Misunderstandings About Precious Coral

The winner of this year’s award was student designer Bingjie Zhao for her magnificent ring design entitled ‘Trawling’. Like the judges I also adored Bingjie’s original and sophisticated design and look forward to seeing her future creations!

Pearls! Pearls! Pearls!

Another of my personal highlights had to be the amazing variety of pearls and pearl jewellery on display. It seemed like there was an ocean of pearls at the show, with wholesale suppliers such as Raw Pearls selling ropes of an amazing variety of pearls, including freshwater, Akoya, and Tahitian, in a sublime array of colours.

The new collection from Ora Pearls

Positioned near the entrance on the ground floor, it was difficult to walk by without stopping to gawp at Yoko London’s divine fine pearl jewellery. At very much the high end of the market, Yoko London’s jewels ooze luxury and many pieces shimmered and sparkled in diamond and white gold settings.

Read more: Saltwater versus Freshwater Pearls

Another fabulous pearl jeweller exhibiting at IJL was Ora Pearls, who was showing a new collection of jewels with a focus on brilliantly large baroque pearls.

New Designers at International Jewellery London

It was very encouraging to see a spotlight on young and emerging jewellery designers at IJL and to see their fresh and unique creations on display. I also really enjoyed getting to speak to these designers and finding out about the inspirations behind their collections. Eloise Kramer told me about her intention to tell a story through jewellery with her nature-inspired pieces, explaining that her colossal resin and copper peach ring should be worn alongside her insect rings to create a scene of creatures swarming around an irresistible fruit.

Eloise Kramer's 'Peach Ring', made from copper and resin, accompanied by one of her 'Insect' rings.

Isla Gilham of Isla Gilham Jewellery was also attracting a lot of attention with her fantastically fun and colourful ‘Jelly Tot Tiara’, which swapped out gemstones for genuine jellies! Another piece which quite literally made me stop in my tracks was a show-stopping necklace from emerging fine jewellery brand Aureliean, which featured a huge sapphire surrounded by diamonds and smaller sapphires and set in 18 carat gold on a double strand of pearls.

 The 'Jelly Tot Tiara' by Isla Gilham Jewellery

Leading Lights Awards & Celebration

As I visited on the first day I was lucky to have been able to attend the drinks reception and Leading Lights Awards, a new addition to IJL for 2019. I found that the awards were a good way to highlight exceptional work from a range of disciplines within the industry – including jewellers, jewellery education providers and bloggers – and to celebrate the achievements of all exhibitors and everyone working behind the scenes to produce such a showcase of international jewellery.

All in all, I had a fantastic experience at International Jewellery London 2019; the variety of interesting jewels on display was a treat for the eyes and I especially loved having the opportunity to speak to designers about their creations and inspirations. I can’t wait to see what next year brings!

Interested in learning more about gemstones? Check out our upcoming workshops.

Are you ready for the Gem-A Conference 2019? Find out more here.

Cover image: Fei Lui Fine Jewellery's 9.35 carat Garnet Ring in rose gold featuring diamonds and rutilated quartz.

Additional Info

Read more...

Gem-A at Hong Kong Jewellery and Gem Fair 2019

Hong Kong Jewellery and Gem Fair is only a few days away and we are getting excited to meet visitors and colleagues from the jewellery and gemstone industries, and of course be dazzled by the many gemmological marvels that will be on show at this world-class event.

We invite you to join us at Booth 3M204 in the Hong Kong Convention & Exhibition Centre from 18-22 September to meet our team, find out about our courses, collect your pre-orders and test your gemmological knowledge with a gem quiz! Although our booth number has changed, we shall be located in the same space as last year (in Hall 3 of the Centre).

We also have an exciting array of additional activities and events planned for our four-day visit to Hong Kong – take a look at our schedule to make sure you don’t miss out!

 

Wednesday 18 September

 

Join us at Booth 3M204!

On the much anticipated opening day of the 2019 show we shall be set up at Booth 3M204, ready and eager to meet you and assist with any questions you may have about becoming a Gem-A student or Member. Please note however that we are not able to carry out membership renewals at any point during the show.

The Gem-A booth at Hong Kong Jewellery and Gem Fair 2018. Look out for our legendary front door at this year's show!

You can also test your gemmological knowledge with our ‘Are You a Gem Detective?’ quiz! Drop by the booth and cast your eye over eight different blue stones and try to identify the gems. If you are new to gemmology you can get some help from our gemmology tutors or use the clues provided.

Read more: Understanding the Value of Sapphires

From Wednesday you can also pick up your pre-orders placed with the Gem-A Shop from our booth at any time during exhibiting hours.

Thursday 19 September

 

Gem-A Meet & Greet

From 11.00am-13.00pm you can meet our CEO, Alan Hart FGA DGA, at the Gem-A Booth and take your photo outside the Gem-A front door!  

Gem-A Gathering

After the show on Thursday 19 September, we shall be holding our third Gem-A Gathering in Hong Kong at the Assaggio Trattoria Italiana in the Hong Kong Arts Centre. Less than a 10-minute walk from the Hong Kong Convention Centre, the Assaggio Trattoria Italiana is a modern Italian restaurant with stunning views over Hong Kong.

We invite you to join us at this relaxed and sociable event, where you will have the opportunity to network and catch up with fellow gemmologists, friends and colleagues from 6.30-8.30pm over drinks and canapés. This is a free event and anyone can attend, but please ensure to register your attendance in advance here.

Friday 20 September

 

Gem Empathy Award: Special Presentation

We would love you to join us at the Gem-A booth at 3.00pm for a celebration of entries from Asia to the 2019 Gem Empathy Award and a special award presentation to one outstanding jewellery design that particularly impressed our judging panel. You can read more about the 2019 Gem Empathy Award and announcement of the overall winner at International Jewellery London 2019 here.

Saturday 21 September

 

Meet your local ATC!

On Saturday we will be joined by representatives from two of our Accredited Teaching Centres (ATCs) in Taiwan. If you are interested in studying one of our courses in Taiwan, this is the perfect opportunity to put any questions you may have to our fantastic ATC tutors.

Read more: Five Things to Consider before Starting the Gem-A Gemmology Foundation Course

From 11.00am-12.30pm Mr Wu Chao Ming will be on the stand ready to answer questions, followed by representatives from Taiwan Gemmological Institute from 3.00-4.30pm.

Sunday 22 September

The Last Day!

We shall be back at Booth 3M204 for the final day of the show. Make sure to make the most of your final chance to meet our team, pick up your pre-orders, find out about our courses and discover how you can get involved with Gem-A!

This year’s show looks set to be packed with a host of seminars, talks and exhibits showcasing some of the world’s most incredible gems and fine jewellery. If you haven’t already, visit the Hong Kong Jewellery and Gem Fair website to book your place for this year’s show. We look forward to seeing you there!

Interested in developing you gemmological knowledge? Take a look at our Gemmology Foundation course.

Do you have a passion for diamonds and want to find out more? Check out our free Introduction to Diamonds webinar scheduled for 10 October 2019.

 

Additional Info

Read more...

ヴィクトリア女王のサファイアとダイヤモンドの宝冠にまつわる歴史

 

ヴィクトリア女王(1819-1901)が彼女の最愛の夫であるアルバート公とともに、宝飾品と色石に愛情を注いだことは有名です。実際、彼女が最も大切にした愛蔵品の一つ ― サファイアとダイヤモンドの宝冠 ― は、1840年に彼女の夫がデザインし注文しました。ここでは、この美しい歴史の1ページとなった物語を紹介します。

アルバート公の生誕200周年を迎えた2019年に、ヴィクトリア女王のサファイアとダイヤモンドの宝冠がロンドンのヴィクトリア&アルバート博物館で永住の地を見つけたことは、まさに運命といえるでしょう。現在、この宝冠は、改修後のウィリアム・アンド・ジュディス・ボリンジャー・ギャラリーの中心にある円筒形のキャビネットにディスプレイされ、再び輝きを放っています。

宝冠は2017年に初めてヴィクトリア&アルバート博物館が獲得したもので、ヴィクトリア女王の最も重要な宝石の1つと考えられています。それは1840年(ロイヤル・ウェディングの年)に夫のアルバート公が心を込めてデザインし、宝石商ジョセフ・キッチングが製作しました。

宝冠にはシルバーにセットしたダイヤモンドと、ゴールドにセットした八角形で子牛の頭の形をしたステップ・カットのサファイア11個がマウントされています。これは、アルバートが結婚式の前日にヴィクトリアに贈ったサファイアとダイヤモンドのブローチに合うようにデザインされていました。パリュール(一揃いのジュエリー)を女王が愛したのは、おそらくこれが始まりです。

そして同じ年に、彼女はダイヤモンドとサファイアのイヤリング、ブローチ、ブレスレットを購入しました。このことは、彼女が素晴らしいジュエリー一式をコレクションしていたことを物語っています!

宝冠のデザインは、ザクセンのラウテンクランツ(上部に葉や花冠をあしらった飾り帯)-アルバート公の紋章- に基づいています。使用されている宝石は以前ウィリアム4世とアデレード王妃がヴィクトリアに与えたジュエリーから取られたものと考えられています。

アルバート公はヴィクトリアのジュエリーに強い関心をもっていました。1843年2月の女王の日記にはこう記されています。「私たちは様々な古いジュエリーを調べ、美しい『パリュール』に加えるためにいくつかの宝石をセットしなおすのに大忙しです。アルバートはセンスがあり、私のジュエリーのためにすべてを手配してくれます」

宝冠はすぐに、1842年のフランツ・クサーヴァー・ヴィンターハルターによる公式肖像画をはじめ、ヴィクトリアの初期の重要な絵画に描かれて後世に伝わっています。これらの肖像画によって、きらめく宝石は彼女の権力とステータスの象徴となりました。

アルバート公の早すぎる死の前には、ヴィクトリア女王が様々に形を変えることができる色とりどりの宝石がついたジュエリーを愛用していたことが知られています。アルバート公の死後、1866年に出席した議会の開会で彼女は重い帝国王冠の代わりにこの宝冠をかぶりました(おそらく、これは宝冠が彼女に自信をもたらしたことを示しています)。

ヴィクトリア&アルバート博物館へ宝冠が収蔵されたことに関して、シニア・キュレーターのリチャード・エッジカム氏は次のように話しています。「ヴィクトリア女王のサファイアとダイヤモンドの宝冠は、彼女の治世の重要な宝石の一つです。アルバート公によってデザインされたもので、彼らが育んだ愛の象徴であり、ヴィクトリアは若い女王として、そして未亡人としてかぶりました。ジュエラーによる傑作というこの贈り物は、ウィリアム、ジュディス・ボリンジャー、ならびに彼らの息子たちのおかげです。そして、それは博物館のアイデンティティの一部になるほど、本当の意味でヴィクトリアとアルバートを連想させるものです」

「多くのサポーターのご厚意によって80もの新しい収蔵品を獲得し貸出いただくことが可能となりました。宝冠は、皆さんに愛されているギャラリーの中心で展示され新しい時を迎えます」

すべての写真のクレジット:ヴィクトリア&アルバート・ミュージアム(V&A)のウィリアム・アンド・ジュディス・ボリンガー・ギャラリー、ジュエリー・ルーム 91-93。

この記事は、Gems&Jewellery Summer 2019 に掲載されたものです。

ヴィクトリア女王、フランツ・クサーヴァー・ヴィンターハルター(1805-73)が描いた後、フランシス・フォスター (1790-1872)による彫版、 パリ、 1846年。 紙に彫版印刷 © Victoria and Albert Museum, London.

Additional Info

Read more...

A Journey into the Blues of Iolite from Tamil Nadu

Gem-A graduate Nishka Vaz FGA shares an abridged version of her stellar Gemmology Diploma project, which focuses on the various colourations of iolites from Tamil Nadu, India.

Few gemstones can boast myriad colours like iolite, one of the most uniquely pleochroic gemstones known. Pleochroism is the name given to the optical effect where a gemstone displays different colours along the different directions in which it is viewed.

One of the pleochroic shades of blue in iolite resembles that of a sapphire so closely that it has earned the title of saphir déau or ‘water sapphire’. When viewed with the naked eye, iolite shows three different pleochroic colours, thus, it is said to be trichroic. The name ‘iolite’ itself is derived from the Greek word ion, which means violet.

An iolite cube cut to display all three pleochroic colours.

As the legend goes, Vikings would carry plates of iolites on board their ships. On cloudy or foggy days with excessive haze and low visibility, it was difficult to mark the exact position of the sun in the sky.

In such instances, navigators would use their iolite plates as a polarizing filter to pinpoint the location of the sun and, using a sundial, determine the geographical north, thus steering the ship back to its correct course; this lead to another common name for iolite: the Viking Compass Stone.

Read more: Understanding the Value of Sapphires

In India, one prime source of these beautiful gemstones are the gem-bearing lithologies of the southern states. Of these, Tamil Nadu has multiple small pockets of gem quality iolites littered around veins found over its vast rolling plains and steep hills. The rocks of southern India converge through a huge network of shear zones and faults to form a lattice of highly metamorphosed rocks.

These rocks guard a variety of different gemstone species. Iolites, rubies, alexandrites, sphenes, sunstones, rock crystals, zircons, aquamarines, and emeralds are just some of the many beautiful gem quality mineral specimens found nestled in these formations.

Iolites from Kiranur, Tamil Nadu.

The Palghat-Cauvery Shear Zone is especially famous for its abundant gemstone mineralization. Iolites, however, may be found sporadically over this large area. Places like Lachmanapatti, Malapatty, Kiranur, Karur, etc., are well known for the presence of gem quality iolite.

Read more: What Should Be in the Ideal Gemmologist's Toolkit?

Obtaining these coveted specimens, however, is not an easy feat. In some cases, they may be mined out of very large pits, or quarried from exposed cliffs. The locals have their own method of extraction: they excavate vertical shafts with a cross sectional area of one square metre extending down to depths of roughly 130 feet. 

When they reach a horizon that appears to be gem-bearing, they then radially extend horizontal tunnels of similar dimensions into the host rock. Multiple shafts and adits eventually form a network of interconnected mining tunnels, not unlike ant colonies.

However, these are only just large enough for a single person to pass though at any given time. If they chance upon a gem-bearing vein lodged between tougher horizons, they only excavate the vein and try to leave the host untouched.

Entrances to iolite mining shafts.

Iolite is a magnesium aluminum silicate, chemically represented as Mg2Al4Si5O18. Iolite is usually found in metamorphic rocks, which are assumed to be formed from the high pressure and temperature alteration of rocks composed of aluminosilicates such as clays, along with an abundance of magnesium rich sediments.

In Tamil Nadu, iolite is found in schists, granulites, and in some cases, even weathered gneisses. They occur along with minerals such as feldspar, sillimanite, biotite, phlogopite, quartz, haematite, diopside, actinolite, etc. which form the matrix of the host rock. Although gem-quality, these iolites do not show proper crystal faces, thus, they are said to have an annhedral habit.

Iolite from Perur, Tamil Nadu.

Tamil Nadu’s reserves being vast, naturally a variation in gem size and quality should be expected. However, a drastic change in size, colour, and overall quality is observed in iolites obtained from across this region. Even accounting for pleochroism, there are iolites found that exhibit colours ranging from pale, washed out blues and violets to deep inky blues that verge on black.

These stones may be so densely included that they almost appear opaque; alternatively, they may be nearly free of inclusions.


Iolite from Arasanatham, Tamil Nadu.

The rare and beautiful bloodshot iolites are also found in Tamil Nadu. They are characterized by an abundance of hexagonal shaped haematite platelet inclusions.

Each platelet behaves as a tiny mirror that reflects light in various directions when randomly oriented. Unlike sunstones, each haematite platelet in a bloodshot iolite reflects a different colour, giving rise to a colourful spangled effect, resembling a colourful glitter confetti.

Spangled effect in bloodshot iolite under 10x magnification.

Even more incredible is the display of chatoyancy and asterism in some of these stones. They may show a faint but clearly visible cat's-eye or star caused by the reflection of light off these haematite inclusions.

A wide range of minerals are found as inclusions within these iolites. Naturally, hexagonal haematite platelets are found abundantly as inclusions in the bloodshot iolites. If these platelets are randomly oriented, they produce a spangled effect.

If they are oriented along a single plane, they produce a cat's-eye effect. If they are oriented along three directions at 120˚, light reflects along a line in each of these orientations to produce an asterisk-shaped reflection pattern of light. This effect is called asterism.

Cat's-eye bloodshot iolite.

An extensive variety of other minerals are also found as inclusions within these iolites. Sillimanite may be seen as long, thin needles which could be oriented in a particular direction, or be randomly dispersed throughout the gem body. Rhombohedral inclusions of calcite are also sometimes observed. Diopside may occur as perfect euhedral dark green to black inclusions.

Read more: Five Things to Consider Before Starting the Gem-A Gemmology Foundation Course

Olive green mineral inclusions from the amphibole group may also be found with a perfectly preserved prismatic habit and deep grooves or striations on their prismatic faces. Well-formed or euhedral inclusions of orange spessartine garnet, or rods of apatite may also occur in the iolites from Tamil Nadu.

Iolite gemstones procured from various localities in Tamil Nadu.

Iolites are fascinating gemstones. The wide range of optical effects that they are capable of displaying is truly captivating – from their unique, visible pleochroism, to the range of shades they can exhibit.

From their sparkly spangled appearance to displaying chatoyancy and asterism, these eccentric gemstones never fail to put on a spectacular display of optical effects. They are incredibly versatile and, in any of their many forms, are certainly a sight to behold.

Interested in becoming a gemmology expert with Gem-A? Read more about our Gemmology Foundation course.

Gem-A also offers a host of one-day workshops on a range of gemmological subjects. Check out our upcoming workshops here.

Cover image: rough iolite specimens. All images are author's own. A bibliography and references are available on request.

Additional Info

Read more...

Understanding the Cat's Eye Effect in Gemstones

Chatoyancy is the gemmological name given to the curious optical effect in which a band of light is reflected in certain cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat's eye. Gem-A's Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.

Chatoyancy is the name given to the ‘cat’s eye’ optical phenomenon which can be seen in certain gemstones. The term ‘chatoyancy’, deriving from the French for ‘shining like a cat’s eye’, denotes the effect that occurs when a bright light is shone onto a rounded, reflective surface and causes light to be reflected back in narrow line perpendicular to the observer’s line of sight. This can be seen by looking at a spool of shiny thread or some cabochon-cut gems.

What Causes Chatoyancy?

Chatoyancy occurs when a band of light is reflected from a series of thin inclusions which are parallel to each other (these may be hollow tubes or needle-like crystals of, for example, rutile or haematite).

 In order to display this effect there must be enough fibres oriented in parallel to the base of the stone. The gemstone also has to be cut as a cabochon to provide the necessary rounded surface for the line of light to be seen.

Read more: The Star of India Sapphire

 As the light moves, or the observer’s eye line moves, the line appears to move too, appearing just like the way a cat’s pupil will look under a bright light.

A chrysoberyl cabochon showing a sharp chatoyant effect. Image by Pat Daly, Gem-A.

 The type of line produced will depend on the types of inclusions; the sharpest, finest line is produced in chrysoberyl, which is the only stone which may actually be called ‘cat’s eye’, without a qualifying species name. Such a stone will possess needle-like inclusions which form an effect known as ‘silk’. As a result, a very definite and sharp chatoyant effect is produced.

Which Gemstones exhibit Chatoyancy?

Many species can exhibit chatoyancy including tourmaline, apatite, beryl, actinolite, demantoid garnet, scapolite, sillimanite and quartz among others but only chrysoberyl can be called simply ‘cat’s eye’, the others must be prefixed by the species name.

Tiger's eye quartz. Image by Pat Daly, Gem-A.

 Chatoyant quartz known as ‘tiger’s eye’ is the only chatoyant gem produced in abundance. It can be found in South Africa and Australia and is particularly popular for use in men’s jewellery.

Cat’s Eye Gems

Cat’s eyes commonly occur in yellow, yellow-brown and greenish-yellow. Some stones, such as cat’s eye alexandrite, can even exhibit colour change which is very rare, extremely expensive and desirable. The chatoyancy is caused by very fine needles which give a bright, sharply defined eye.

A cat's eye alexandrite appearing green in daylight and purple-red under incandescent light. Images by Charlie Bexfield, Gem-A.

 Cat’s eye can also show a mesmerising ‘milk and honey’ effect where one side of the line is milky and the other translucent. As light moves across the surface the bands part and merge like the blinking eye of a cat. This particular effect has led some to believe that the stone possesses supernatural qualities.

Read more: Exploring the Varieties of Quartz

 At 8.5 on the Moh’s scale of hardness, cat’s eye is a hard and durable stone and is especially popular in Japan for men’s jewellery due to its rarity.

 Cat’s eye was also a very popular stone for engagement rings in Victorian times, when it was known as chrysolite. Another old name allocated to this stone is cymophane which means ‘wave’ and ‘appearance’ because of its optical effects.

 Chatoyancy is an interesting and unusual effect worth looking out for to add interest to either a jewellery or mineral collection. What’s more, it doesn’t have to be expensive; actinolite, sillimanite and apatite can be found quite easily, and cat’s eye quartz is very popular and obtainable.

Do you have a passion for gemmology but don't know where to start? Why not consider trying one of our upcoming Introduction workshops?

Interested in developing your knowledge on gemmology? Take a look at our Gemmology Foundation course.

Cover image: A yellow chrysoberyl cabochon showing a cat's eye effect. Image by Pat Daly, Gem-A.

Additional Info

Read more...

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Locked away in the vaults of Gem-A HQ in London is an assortment of breath-taking treasures that form Gem-A’s Gemstones and Minerals Collection. Here, Gem-A Gemmology Tutor, Pat Daly FGA, offers us a glimpse at some of the more unusual items in the collection.

Decades of collecting, bequests and acquisitions have led to the creation of the Gem-A Gemstones and Minerals Collection, which includes red beryl, banded fluorite, citrine, gypsum, peridot, zoisite and many more specimens that are worthy of conversations in their own right. Now, thanks to fantastic photography by Gem-A’s Henry Mesa, we can share some of these unique gemstones with you.

An example of a coral skeleton. Image by Henry Mesa, Gem-A.

 

Coral Skeleton

This example shows a classic coral skeleton: a branching, porous supporting structure produced by a colony of marine invertebrates.

Read more: Saltwater versus Freshwater Pearls

Precious coral, on the other hand, is a more compact and less porous material with a deeper saturation of colour. The sale of coral, like all organic gem materials, is subject to controls, which are designed to preserve marine faunal diversity and maintain stocks for the future.

A rough jadeite specimen. Image by Henry Mesa, Gem-A.

Jadeite

Also part of the Gem-A Collection is this cobble of rough jadeite which has been polished on one side to reveal the bright lustre modified by a dimpling, which is common on polished jadeite. This has a pleasing variation of white, green and lavender colours and a granular structure.

Read more: Jade and its Importance in China

 Jadeite is a polycrystalline gem material, composed of many small interlocking crystals. This structure gives it great strength and its resistance to breakage means that this gemstone can be made into delicate carvings, which are highly valued in China.

Emerald in Pyrite

This specimen (see cover photo) showcases well-formed crystals of emerald from Muzo, Colombia, in pyrite. Both minerals are formed from hot aqueous fluids circulating at high-levels and under great tectonic pressure in the earth’s crust.

A rough tanzanite crystal. Image by Henry Mesa, Gem-A.

Tanzanite

The mineral zoisite is well-known as an abundant compound in some rock types, but it was of little significance to the gemstone trade until 1967, when tanzanite was found in a relatively small, two kilometre-wide belt in Tanzania. This well-formed crystal of tanzanite now forms part of the Gem-A collection, along with other cut and faceted tanzanites with deeper purple tones.

A box of 19 gemstones from the Anderson Collection, including a 30.82 carat beryl (centre). Image by Henry Mesa, Gem-A.

The Anderson Collection

This historically significant collection of gemstones was once owned by Gem-A founding father, Basil Anderson. In the 1980s, the entire collection of exceptional quality gems was destined for auction and risked being lost to the Association forever.

 

Gems from the Anderson Collection, including a 39ct aquamarine specimen (centre). Image by Henry Mesa, Gem-A.

 

Recognising this, an anonymous donor purchased the entire collection and donated it to Gem-A in 1986. Highlights include a specimen box containing 17 gemstones, including a 30.82 carat beryl, and a second box containing nine gems with a 39 carat aquamarine.

Cover image: Muzo emerald crystals in pyrite. Image by Henry Mesa, Gem-A.

This article was originally published in the Autumn 2018 issue of Gems&Jewellery (Volume 27, No.3).

Are you passionate about gemstones but new to the science of gemmology? Why not try one of our Introduction workshops.

Interested in taking your gemmological education further? Take a look at our Gemmology Foundation course

Additional Info

Read more...

American Gemstones: Yogo Sapphires from Montana

Native to Montana, the sought-after Yogo sapphire was first discovered accidentally by a gold miner in the 1890s. Elizabeth A. Gass FGA, a Gemstone Advancement and Education Coordinator at JTVguides us through the gemmology and origins of the Yogo sapphire and explains what makes this dazzling American gem so special.

Montana has so much mineral wealth that one of its nicknames is ‘The Treasure State’. Gold is what first drew settlers to its vast landscape but, in the gemmological community, Montana is most well-known for one of its more colourful treasures: sapphire. There are a few mines that have produced sapphires over the years, but the most desirable material is from Yogo Gulch. 

A Surprising Find

The story of the discovery of Yogo sapphires is much like the discovery of many American sources of gemstones; no one was really looking for them. In 1895, a miner by the name of Jake Hoover discovered gold along a bench (flat area on the side of a mountain) east of Yogo Creek. Thinking that there was the possibility that the deposit could yield significant amounts of gold, he searched for investors. 

They raised somewhere around $40,000 to invest in the mining operation, $38,000 of which was used to construct a ditch and flume to bring water to the mine. It extended for several miles and evidence of it can be found today. Luck was not on their side and after a year of mining they came away with only 40 ounces of gold, worth around $700. Miners had been noticing ‘blue pebbles’ in their sluice boxes since mining took off in the area but everyone just discarded them in their search for gold.

 

A cut and polished Montana sapphire.

Hoover was different, he collected the small blue stones in a cigar box until it was full. He then sent them off to Tiffany & Co. to see what they were and if they were worth anything. Hoover received quite the surprise when George F. Kunz of Tiffany & Co. identified the pebbles as sapphire. Tiffany & Co. bought the cigar box full of rough for $3,750 and Kunz declared the stones “the finest precious gemstones ever found in the United States” due to their exceptional colour and clarity. 

Learn more: Sapphire for Those Born in September

Realising that the sapphires could be the remedy to their financial problems, Hoover and his partners set out to find the “mother lode”. Another prospector stumbled upon the source of the sapphires when he found a fissure filled with soft material in a limestone outcrop. When he washed some of the material, he immediately found sapphires. Hoover and company quickly identified the vein as the source of the sapphires. It did not take long for claims to be staked along the five-mile length of the vein. Eventually, 33 claims, all mining sapphire, were established.

Origins of the Yogo Sapphires

The famous Yogo sapphires would never have existed without the geological processes that occurred in the Yogo mining area. They are found in a system of igneous intrusions called dikes. There are six main dikes that form the centre of the Yogo sapphire deposit. All are composed of a type of igneous rock called a lamprophyre. A lamprophyre is a dark igneous rock with porphyritic texture, meaning that there are larger crystals in a fine-grained matrix.

These dikes intruded into the predominantly limestone country rock. The limestone had previously undergone dissolution processes that created karst topography in the form of channels, caves, and sinkholes. The lamprophyre dikes followed the natural topography during emplacement. The main dike, which is where the bulk of the mining for Yogo sapphires has taken place, is un-weathered compared to the other dikes. The weathered dikes are altered to clay minerals, often yellow to reddish brown in colour. 

A tie or lapel pin set with a Montana sapphire.

One dike that was discovered does not bear sapphire. It is a mafic lamprophyre whereas the others that do bear sapphire are all ultramafic. This means that the sapphire bearing rocks are made of less than 45% silica. Because the other dikes are altered, the main dike is the most appropriate dike to study and infer the emplacement conditions of the Yogo sapphire deposits. It is also the only dike to have significant research available.

The main dike has a limited amount of contact metamorphism with the surrounding rock which indicates a quick emplacement and cooling process. The magma that made the lamprophyres is thought to have originated in the upper mantle. The magma moved through existing metamorphic rocks, rich in aluminum, partially melting and heating them. The sapphires formed as a reaction between the magma and the aluminum-rich rock in what is called a peritectic reaction. 

Learn more: Understanding the Value of Sapphires

Peritectic reactions occur between a solid (country rock) and a liquid (lamprophyre) at certain temperatures and compositions resulting in a new solid (sapphire and other minerals). The sapphires did not crystallise from the lamprophyre, but the intrusion caused their formation. Pieces of the country rock that now contained sapphire were incorporated into the magma as xenoliths. The magma melted many of these xenoliths and incorporated the elements into itself. Even the sapphires were partially absorbed with some showing a spinel reaction rim or etching and pitting. There are a few theories of how the sapphires came to be in these igneous bodies, but this is one of the more recent that correlates well with the known data.

Visual Properties of the Yogo Sapphire

The unique geology of the Yogo mining area leaves an imprint, not just on the geologists and miners that know the area, but also on the sapphires themselves. They are known for their even colour, lack of colour zoning, and are fairly-free of inclusions. Their quality as cut stones is also increased due to their high lustre and brilliance. They are typically a ‘cornflower blue’ colour but can also be found in shades of violet or purple. Typically, it is difficult to distinguish sapphires from different deposits because of how physically similar they are. 

Yogo sapphires, on the other hand, are wholly unique. Their origin can be considered igneous, instead of metamorphic, like most sapphires. They have unique inclusions and distinct trace element concentrations that make them readily distinguishable from sapphires mined in other locations.

Learn more: The Siren of Serendip Blue Sapphire

Some of the more notable and recognizable inclusions include negative crystals of carbonates or analcime, pyrope-almandine-grossular garnet crystals, elongated dark rutile crystals with or without tension fractures and melt inclusions with contraction halos. There is also a notable lack of silk in Yogo sapphires. If it is present, which is extremely rare, it is a minute amount which is called ‘rutile dust’. The trace element concentrations distinguish them by their generally elevated magnesium and titanium content. Some Sri Lankan sapphires can overlap the lower end of the Yogo sapphire range, but they always have a much higher iron concentration than Sri Lankan material. 

The United States may have vast wilderness and incredible natural wonders, but it is some of our smallest treasures that have captured mankind’s attention. Sapphires from the Yogo Gulch of Montana are no different. This unique formation environment provided a distinct chemistry and readily identifiable inclusions. The Yogo sapphire’s accidental discovery and all the subsequent successes and failures of different mining ventures gives this precious stone an incredible story.

Title image: A faceted Montana sapphire alongside a pearl tie or lapel pin set with a Montana sapphire.

Complete bibliography and references available upon request. All images courtesy of JTV.

This blog was originally published as part of the article, ‘One Nation, Many Gemstones’ in Gems&Jewellery Vol. 29, No. 1.

Discover more about coloured gemstones in Gem Knowledge, part of the Gem-A Gem Hub. Find out more about JTV here.

Interested in becoming a qualified gemmologist? Take a look at our Gemmology Diploma and Gemmology Foundation courses.

Additional Info

Read more...

American Gemstones: Benitoite from California

In 1907, by the headwaters of California's San Benito River, a new blue gemstone was discovered which was to be named benitoite. Here, Elizabeth A. Gass FGA, Gemstone Advancement and Education Coordinator at Jewelry Television (JTV), takes a closer look at this all-American gemstone. 

The United States is a geographically vast and geologically diverse nation. It comes as no surprise that it would host some of the most sought-after gemstone species in the world. Some of these are wholly unique, while others are some of the finest quality known to man. One mineral, with a rather unique discovery story and geological occurrence, is benitoite. 

Benitoite was discovered in 1907 by James Marshall Couch, a failed melon farmer turned prospector, near the headwaters of the San Benito River in the mountains of the Diablo Range (Wilson, 2008). 


A benitoite crystal in matrix. 

The type locality of benitoite, the New Idria district, San Benito County, California, is also the only location that has produced facet grade benitoite. Out of less than 10 known locations around the world, only three are outside of California (Japan, Australia and Arkansas, USA), and only two mines in the New Idria district, the Benitoite Gem Mine and Junila claim, have produced any facet grade material. Of those two mines only one ever produced faceted material in commercial quantity, the Benitoite Gem Mine. It was also the original mine opened at the site of discovery (LaursRohtert & Gray, 1997).  

The Benitoite Gem Mine operated in some capacity from 1907–2005 under several different names and owners. The site was reclaimed in 2005 and sold (Wilson, 2008). It is currently open as a small pay to dig operation. 

The Race to Name Benitoite

When benitoite was first discovered it was thought to be blue diamonds by Mr. Couch, who showed them to his financier Roderick William Dallas. Dallas immediately sent Couch back up the mountain to stake a claim.  

All the formal paperwork was filed within eight days of the initial discovery, but no one involved knew exactly what kind of blue gemstone they were looking at. Dallas asked Leland Barnes Hawkins, Sr., a mine engineer and friend to look at some of the rough crystals and take them to a jeweller in Los Angeles to determine if they had value. 

Learn More: Red Beryl from Utah

The jeweller declared them to be a form of blue obsidian. Then another jeweller identified them as spinel because of their colour. A faceted sample, labelled as a spinel from California, was sent to a jewellery firm in San Francisco. The head of their diamond department studied the stone and realised that the gem wasn’t anything he had seen before. He purchased the stone for $40 and sent it to a friend, Davis Louderback, who was an Associate Professor of Geology at the University of California, Berkley. 

Benitoite in matrix.

After concluding that the stone was neither spinel nor sapphire, he asked for more material, preferably crystals, to confirm that this was a new mineral. After backtracking the gem to its source, it took Louderback only a few days, with the new samples, to confirm that Couch and company had a new mineral on their hands. From there, a visit to the mine was required so that Louderback could write a formal bulletin on the discovery. 

Word of a possible new gemstone reached Tiffany & Co.’s George F. Kunz, who sent a telegram asking for more information on the discovery. From there the race was on to see if Louderback could complete his research before Kunz wrote his own proposal. Others involved with the discovery pushed Louderback to tirelessly work on his proposal because they wanted their Californian gem to have a “California name”. Louderback succeeded and on July 30, 1907 his report was published, officially naming the new mineral ‘benitoite’ (Wilson, 2008). 

Benitoite's Geological Origins

Benitoite’s discovery could have never happened without the unique geology of the New Idria region. The New Idria region is predominantly composed of a serpentinite body that was tectonically emplaced into the surrounding sedimentary and metamorphic rocks (LaursRohtert & Gray 1997). When two tectonic plates collided, one dipped below the other in a process called subduction. This creates a massive amount of heat and pressure. Because of this the serpentinite rose through the overlying rocks due to its relatively lower density as a plume.  

As it rose it captured some of the overlying rocks, namely those of the Franciscan Complex, which is composed of a wide variety of rocks like basalt, greenstone, chert, limestone, sandstone and blueschist. The main constituents are graywacke sandstone and blueschist, but all the mentioned rock types can be found and are highly folded and faulted in the Franciscan Complex. The serpentinite is composed mainly of chrysotile-lizardite serpentinite and minor antigorite serpentinite (Tsujimori, 2007). With these randomly interspersed pieces of the Franciscan Complex the geochemistry varies widely from one area to the next.  

Rough benitoite (right) and a cut and polished benitoite gemstone (left).

Once these rocks were in place, a later intrusion of syenite (intrusive igneous rock similar to granite but containing less than 5% quartz) in the southern portion of the serpentinite body took place. This caused hydrothermal alteration to occur in localised areas of the serpentinite and its tectonic inclusions (Tsujimori, 2007).  

Learn more: Sweet Home Mine Rhodochrosite from Colorado

The benitoite is solely found in the hydrothermally altered zones of the blueschist and, even then, only when the amphibole and pyroxene show recrystallisation; there is dissolution of albite and pervasive infilling of natrolite in the veins (LaursRohtert & Gray, 1997). Even then, the benitoite usually only forms where the veins narrow or terminate and it is always coated by natrolite, which formed in the later stages of alteration. Geologically this is a very narrow window for the growth and formation of benitoite and other accessory minerals like neptunite. 

Benitoite's Gemmological Properties

Because of benitoite’s rarity in the world and its very specific growth requirements, it should come as no surprise that its crystallographic properties are also unique. It is a barium titanium silicate (BaTiSi3O9) that forms in the trigonal portion of the hexagonal crystal system. The common form that it crystalizes in is a ditrigonal-dipyramidal habit, which is the most complex form of a trigonal crystal (Louderback & Lawson). This form is so rare, in fact, that it took 77 years for a natural occurrence of this morphology to be found after it was predicted in 1830 (LaursRohtert & Gray, 1997). 

Crystals are typically smaller than one centimetre with many of them being highly included. Gemstones are typically very small due to the flat habit of the crystals, with strong colour zoning and inclusions of amphibole and pyroxene.  

Examples of cut and faceted benitoite in a range of hues.

Cut stones are typically smaller than one carat but many stones larger than 4ct have been produced over the years with the largest cut benitoite weighing in at 15.42ct (LaursRohtert & Gray, 1997). Don’t let the relatively small size of benitoite fool you though; it can come in a beautiful range of colours from blue to slightly purplish-blue, or white to colourless, with very few stones being naturally pink.  

Most stones are not treated in any way, but an orange colour can be caused in some stones through heat treatment. Even though the dispersion is sometimes masked by its deep colour, it is in fact higher than that of a diamond. This sets it apart from many other similarly coloured gemstones.  

Learn more: Yogo Sapphires from Montana

The state gemstone of California is as American as a gemstone can be. It comes from unique geological circumstances, has a rare crystalline structure, and has an incredible story of its discovery by a humble melon farmer turned prospector, looking to improve his family’s lot in life, through determination and a lot of luck. Considering all this, benitoite is truly one of the greatest American treasures. 

This blog was originally published as part of the article, ‘One Nation, Many Gemstones’ in Gems&Jewellery Vol. 29, No. 1. 

Discover more about coloured gemstones in Gem Knowledge, part of the Gem-A Gem Hub. Find out more about JTV here. 

Interested in becoming a qualified gemmologist? Take a look at our Gemmology Diploma and Gemmology Foundation courses. 

Main image: An example of benitoite in matrix. Complete bibliography and references available upon request. All images courtesy of JTV. 

 

Additional Info

Read more...

Understanding Specific Gravity and Why it is Vital to Gemmologists

Specific Gravity is a vital test that all gemmologists need to be aware of. Here, Gem-A Gemmology Tutor, Dr Juliette Hibou FGA, explains the concept of specific gravity and how it is used by gemmologists to aid in gemstone identification.

Different gemstones have different densities; this property is used to differentiate between materials. In gemmology, we use specific gravity (SG), or relative density, as a test to identify gemstones.

In 1817, in his Traité des caractères physiques des pierres précieuses, the French mineralogist Valentin Haüy was the first to use specific gravity as a systematic test to identify gemstones. He created a table with SG constants for all the varieties of gemstones listed in his treatise.

Learn more: What Should Be in the Ideal Gemmologist's Toolkit?

Although testing for SG can be thought of as a bit fiddly and impractical, it is still a test that can be very relevant when trying to identify a gem material. SG is a ratio (there it has no unit) which is calculated by dividing the weight of a material in air by the weight of an equal volume of water. It can be speculatively measured by the heft of the material: a material with a higher heft feels heavier.

Eureka! The History of Specific Gravity 

In Ancient Greece, King Hieron II of Syracuse wanted to ascertain whether a crown he had commissioned was made of pure gold. Archimedes (c.287-212 BC), a mathematician and inventor, was set the task, and he established the principle of buoyancy, famously while taking a bath: any object wholly immersed in a liquid experiences a force equal to the weight of the volume of liquid it displaces.

Archimedes in his bath realising that a body displaces fluid equivalent to its volume. This means that, regardless of shape, objects of equal density and weight displace the same quantity of fluid. Archimedes used the principle to determine if the crown was of pure gold. As the crown displaced more water than the same weight of pure gold, it proved that it was alloyed with other metals. Public Domain. 

Archimedes then compared the volume of water displaced by equal weights of gold and silver and realised that silver displaced more water than gold (as being less dense it had a larger volume). He then immersed the crown and after a series of experiments and calculations managed to prove that the crown was made of gold mixed with a certain amount of silver - and the jeweller who made the crown was punished for his wrongdoing.

The Shift Towards Hydrostatic Weighing

As a result of this principle, centuries later, the Italian mathematician and astronomer Galileo (1564-1642) devised the following formula to calculate the SG of a material, also known as hydrostatic weighing:

Weight in air / weight of an equal volume of water = weight in air (A) / weight in air – weight in water (A-W) (which equals to the weight of the same volume of water)

Or more succinctly put: SG = A / A-W

Aged 22, he wrote a short treaty on La Bilancetta or ‘little balance’ to explain the workings of a scale he designed to weigh objects in air and in water accurately, and as a result identify the material based on their SG. His two-pan scales have been used and perfected for centuries.

Galileo Galilei, who devised a formula for calculating specific gravity in the 16th Century. Public Domain. 

 

How to Calculate Specific Gravity

Today we calculate SG with hydrostatic weighing. It can be used for both loose fashioned and rough materials. This test is very useful for rough materials that cannot be tested on the refractometer if they have no flat polished facet.

Equipment needed:

  • Gemmological scales, i.e. single pan scales, preferably digital and measuring in carats.
  • A hydrostatic apparatus, made of a bridge and a small glass beaker, with a small metal basket or coiled wire forming a little basket in which the gem can be placed.
  • Water that has been boiled or distilled and cooled down to 4 degrees. In order to be able to use the SG table, measurements need to be done in the same conditions, with water that has the same density.

Method:

  1. Set up the hydrostatic weighing apparatus on the scale. Ensure that the small basket is not touching the sides of the beaker and is fully submersed in the water. Make sure that the scale is on a stable surface and that no one taps the table or walks past while measuring the gemstone as scales are sensitive. Any movement can alter your results.
  2. Clean the material being tested.
  3. Measure the weight of the gem in air. Take the scale and place the stone one the scale below the apparatus. Record your result, including two decimal places. Note that no units are being recorded as SG is a ratio without units.
  4. Next, measure the weight of the stone in water. Take the scale and, using tweezers, gently place the gemstone in the metal or wire basket. Record the result including two decimal places.
  5. Finally, apply the SG formula (A/A-W), dividing the weight of the gem in air (A) by the weight of the gem in air minus the weight of the gem in water (A-W).
  6. Check your result against the SG constant table.

Caveats

An SG test can only be performed on loose gemstones. Impurities and inclusions, which do not have the same SG as their host crystal, will interfere with the result, and limit the usefulness of SG testing. Bubbles of air trapped on the surface of the gem, in a drill hole, or inside surface reaching fractures will also make measurement less accurate.

Composite gemstones (made of two or more materials) such as opal doublets, soudé gemstones and garnet topped doublets cannot be tested. Treated materials won’t give accurate results, especially coated and fracture-filled materials.

Why is there a Specific Gravity range for each gem?

A gem material will always have a range of SG values because a single gem variety is rarely pure and will have a varied chemical composition with inclusions and fractures that will slightly influence its SG. For instance, a spinel has an SG that will range from 3.58 to 3.61, depending on its composition. The SG of a Verneuil synthetic spinel is slightly higher at 3.61 to 3.67 (the range is due to the different colouring elements added).

Uses of Specific Gravity

Calculating the SG of a gemstone can be extremely useful for gemmologists; an SG test can help to identify certain gems, and also to estimate the weight of gemstones already set in jewellery. 

Differentiating between similar materials

Measuring SG is a test useful for differentiating between two similar-looking gemstones, for example: blue iolite (1.54 to 1.56), a tanzanite (1.69 to 1.70) or a blue sapphire (3.8 to 4.05); or red tourmaline (3.0 to 3.1), red spinel (3.58 to 3.61) and ruby (3.80 to 4.05).

Note that the SG for a red almandine garnet (3.80 to 4.2) overlaps with that of a ruby and therefore this test will not be enough to distinguish these two gemstones. It is important to remember that SG is not a diagnostic test.

Learn more: 5 Things to Consider before Starting the Gem-A Gemmology Foundation Course

However, if observation and no other tests are conclusive, SG testing will narrow down the range of possibilities, especially when testing for minerals less commonly used in gemmology.

Weight Estimation

Knowing the SG of a material can also be very useful when trying to estimate the weight of a gemstone set in jewellery and estimate its price as a result. Once you know the identity of the gemstone and its SG you can estimate its weight without unsetting it.

Weight estimation is not as accurate as weighing a loose gem on a scale but most of the time it is not practical to unset a gem to weigh it. This method is the best way to get a weight estimate.

Discover more about the science of gemmology in Gem Knowledge, part of the Gem-A Gem Hub. Interested in becoming a gem expert? Take a look at our Gemmology Foundation and Gemmology Diploma courses.  

Cover image: Public Domain.

Additional Info

Read more...
Subscribe to this RSS feed