Journal Digest: Radiocarbon Age Dating on Natural Pearls

Guy Lalous ACAM EG is on-hand to summarise some of the more in-depth articles from Gem-A's The Journal of Gemmology. Here, he explores an article on radiocarbon age dating of natural pearls from the Winter 2017 issue. 

This article describes how radiocarbon age dating can be adapted to the testing of historic pearls. The authors, Michael S. Krzemnicki, Laurent E. Cartier and Irka Hajdas, have developed their sampling method so that radiocarbon age dating can be considered as quasi non-destructive. The refined sam­pling process allows researchers to work with tiny amounts of nacre powder (~2 mg) taken from a drill hole without any damage to the outer surface of a pearl. In this article, pearls originating from a historic shipwreck were submitted to radiocarbon age dating. 

A small selection of pearls (approximately 2–8 mm diameter) from the Cirebon shipwreck was investigated for this study.
The pearls are shown on a historic map of the Java Sea, where the shipwreck was discovered. Photo by Luc Phan, SSEF.

The 10th century Nan-Han shipwreck was discovered accidentally in 2003 off the northern coast of Java, Indone­sia, near the city of Cirebon. The excavation of the ancient merchant vessels produced Yue ceramics, glassware and Chinese coins dating from the 10th century CE, jewellery, loose gemstones and also a number of carved gastropod shells and pearls. The thousands of glass fragments, and several unbroken blue and green glass objects found in the Cirebon shipwreck, undoubtedly originated from the Islamic Middle East. 

This indicates extensive trade in Southeast Asia along maritime routes at that time, which the Cirebon merchant vessel was a part of. This also supports a Persian Gulf origin for the pearls. The partly abraded and brown-to-grey altera­tions around the drill holes of these pearls suggests that they might have been in use for some time, strung on strands or set with metal lin­ings in jewellery before they sank in the vessel with the rest of its cargo. The coins and artefacts provided good evidence for a 10th century age of the shipwreck. 

What about X-radiography?

X-radiography is an imaging technique. X-rays are located beyond UV in the electromagnetic spectrum, where they have even shorter wavelengths and greater energy. Materials of low atomic weight allow x-rays to pass through easily and, therefore, appear dark on x-ray film, and those of high atomic weight block x-rays and appear white.

What about X-ray luminescence Computed Tomography?

X-ray luminescence is an emerging technology in X-ray imaging that provides functional and molecular imaging capability. This emission-type tomographic imaging modality uses external X-rays to stimulate secondary emissions, which are then acquired for tomographic reconstruction. This modality surpasses the limits of sensitivity in current X-ray imaging. 

What about EDXRF?

X-Ray fluorescence analysis using ED-XRF spectrometers is a commonly used technique for the identification and quantification of elements in a substance.

What about X-ray computed microtomography?

Seeing inside a material object, in three dimensions, is often crucial for proper characterisation, so that the link between microstructure and properties can be made. Micro-computed tomography or "micro-CT" is X-ray imaging in 3D on a small scale with massively increased resolution. It really represents 3D microscopy, where a very fine scale internal structure of an object is imaged non-destructively.

Fourteen pearls were analysed routinely by X-radiography and X-ray lumines­cence, as well as by EDXRF spectroscopy. Four of them were selected for X-ray computed microtomography (micro-CT) analysis. 

These 14 pearl samples (71742_A–N; approximately 2–8 mm diameter) from the Cirebon shipwreck were examined for this study.
They are partially abraded around their drill holes and show some brown to grey colour alterations. Photo by Luc Phan, SSEF.

Based on their X-radiographs, trace-element com­position and lack of luminescence to X-rays, the samples studied for this report were all saltwater natural pearls. The separation of natural from cultured pearls is mainly based on the interpretation of their internal structures. The radiography and micro-CT scans revealed that their internal structure main­ly consisted of fine ring structures typical of natu­ral pearls.

What about radiocarbon age dating?

The collision of high-speed neutrons produced by cosmic ra­diation with the nucleus of nitrogen results in the capture of a neutron and the expulsion of a pro­ton, thus transforming the 14N isotope into the radionuclide 14C. The radiocarbon, present only in trace amounts in the atmosphere, combines with atmos­pheric oxygen and forms radioactive carbon di­oxide, which is then incorporated into plants by photosynthesis and subsequently into animals via respiratory and metabolic pathways. As a consequence, the radiogenic 14C is incorporated into the endo- or exoskeletons (e.g. bones or shell structures) of animals.

After death, the lifelong exchange of carbon with the environment suddenly stops, resulting in a slow radioactive decay of 14C in the dead plants and animals. By measuring the ratio of radiogen­ic and stable carbon isotopes (14C/12C), it is thus possible to determine their age. The so-called half-life of 14C (that is, the time at which only half of the original 14C is still present in a sample and, as such, represents the constant rate of de­cay over time) is about 5,700 ± 40 years.

What about MICADAS?

The MICADAS is a mini carbon dating system through accelerator mass spectrometry. It is a two-step process. The first part involves accelerating the ions to extraordinarily high kinetic energies, and the following step involves mass analysis. The system allows radiocarbon analyses of ultra-small samples with great accuracy in only a couple of hours’ time. 

A pearl is a calcium carbonate (CaCO3) con­cretion formed by bio mineralisation processes in a mollusc—very much the same processes as for shell (exoskeleton) formation. As such, pearls (and shells) contain carbon, mainly the stable iso­tope 12C (as well as 13C) but also a small fraction of radiogenic 14C. The carbon used for the bio mineralisation of pearls and shells mainly originates from two very different carbon pools: (1) oceanic dissolved inorganic carbon; and (2) respiratory CO2, mainly stemming from food metabolism.

As such, the so-called marine reser­voir age effect may distinctly affect the resulting 14C ages of shells and pearls, especially in areas with upwelling of ‘old’ water.  Hence, a correction is required to take into account the geograph­ic location of the sample. The 14C/12C ratio was measured on three samples using the Mini Carbon Dating System (MICADAS).  

The calculated 14C age BP was corrected by apply­ing a marine reservoir correction that was based on values for the Java Sea location. These were estimated (mean-weighted) based on 10 data points in the vicinity of the sampling site. The result corresponds approximately to the end of the 10th century, which cor­relates well with the age stipulated for the coins, pottery and other artefacts found in the shipwreck. 

 Age determination can support evidence of historic provenance in the case of antique jewellery and iconic natural pearls. It can also be used to identify fraud in cases where, for example, younger pearls are mounted in historical jewellery items, or have been treated so that they appear older rather than having been farmed during the 20th century. 14C age dating can be used to obtain evidence to support a decision whether a pearl is of natural or cultured formation. This is because methods to commercially cultivate pearls from certain mollusc species only began during the 20th century.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Radiocarbon Age Dating of 1,000-Year-Old Pearls from the Cirebon Shipwreck (Java, Indonesia) by Michael S. Krzemnicki, Laurent E. Cartier and Irka Hajdas 2017/Volume 35/ No. 8 pp. 728-736

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The Fascinating History of Antique Turquoise Jewellery

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Birthstone Guide: Garnet For Those Born In January

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Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

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Understanding the Cat's Eye Effect in Gemstones

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Jade and its Importance in China

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Highlights of Gem-A Conference 2019

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The Gem-A Conference is always the highlight of our gemmological calendar! If you didn’t manage to make it, we’ve put together a few of the highlights from this year’s event to fill you in on what you missed, and whet your appetite for Gem-A Conference 2020!

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Buying Guide: Saltwater versus Freshwater Pearls

Treasured the world over for their timeless elegance, lustre and iridescent multi-tonal colours, pearls have been a favourite of high-end jewellers and collectors alike for hundreds of years. 

The majority of gem-quality pearls are produced by bivalve molluscs (an animal whose shell has two hinged parts), but they can also be produced by gastropods (a single shell) which are more rare and may come at a higher price. A pearl is created when an irritant agitates the body of a mollusc and initiates the growth of nacre, a substance that is created by platy crystals of aragonite (a calcium carbonate mineral) held together by an organic compound known as conchiolin.

Well-shaped, naturally occurring specimens can be very rare and highly expensive – in 2017, a natural pearl and diamond drop pendant sold for US$1,452,500 at a Bonhams auction. Cultured saltwater and freshwater pearls are far more abundant and can be cultured to produce excellent lustre in a variety of colours and sizes. But what are the main differences between saltwater and freshwater pearls, and what are the key characteristics that make the highest quality and most desirable specimens?

Saltwater Pearls

Saltwater pearls are produced by oysters found in the sea and generally only a single pearl per shell is found. As natural saltwater pearls are extremely difficult to come across, cultured specimens make up the vast majority on the market. Cultured saltwater pearls are farmed with a bead nucleus made from shells of freshwater molluscs (mussels) as these varieties tend to have thicker shell sections which are ideal for fashioning into a sphere.

3.5-5.9mm Akoya pearls, 3.979cts of diamonds, set in 18ct white gold. From the Yoko London Raindrop Collection. Image courtesy of Yoko London.

Akoya Pearls 

Kokichi Mikimoto perfected the process for culturing pearls in Japan in the 1890s, utilizing Akoya pearl oysters. Akoya pearls are characterised by their very bright lustre and usually white body colour, often enhanced by bleaching. Pink, silver, blue and yellow shades can also occur naturally. Akoya pearls will normally have a diameter of 6-8 millimetres.

Read more: Pearls for Those Born in June 

Grey and black saltwater pearls are highly desirable and have been known to fetch stellar prices at auction. The Cowdray pearl necklace, a string of 42 natural grey saltwater pearls once owned by Viscountess Cowdray, broke the auction record for the sale of a pearl necklace when it sold for US$5.3 million at Sotheby’s Hong Kong in 2015. 

8.5-13.8mm South Sea, Akoya and Tahitian pearls, 18ct white gold clasp. Part of the Yoko London Ombre Collection. Image courtesy of Yoko London.

South Sea Pearls 

South Sea pearls can also be cultured, and although natural specimens would be infinitely more valuable. Cultured South Sea pearls have been known to reach astronomical prices at auction. In 1992 a strand of 23 Australian South Sea pearls sold for $2.3 million at Sotheby’s Geneva. Among the larger of saltwater varieties, they will typically reach 11-14mm in diameter but in some cases they can grow much larger. Part of the desirability of South Sea pearls may be the iridescent gold and silver hues they acquire from the silver-lip and gold-lip molluscs in which they grow.

Tahitian Pearls 

Cultured in a black-lip oyster, Tahitian pearls have a darker nacre with surface iridescence and overtones of peacock, blue, green, purple and gold. Tahitian pearl quality is regulated by the French Polynesian government exerting tight controls to ensure consistent, quality material reaches the market.

12-13mm Tahitian pearl, 0.49ct of diamonds, set in 18ct white gold. From the Yoko London Classic Collection. Image courtesy of Yoko London.

Cortez Pearls 

Originating from rainbow-lipped molluscs found in the Sea of Cortez in Mexico, Cortez Pearls are dark in body colour and display a fabulous rainbow iridescence on the surface. Furthermore, these pearls are somewhat unique as they are the only variety which can show an unusual red fluorescence when exposed to long wave ultra violet light.

Fiji Black Pearls 

Another highly desirable variety of saltwater pearls are Fiji black pearls. Among the rarest types of pearls in the world, they are also a relatively new variety, having first entered the industry in the 1960s. Since then, their introduction by J. Hunter Pearls in 1999 has increased and expanded their market. As a result, we can now see Fiji pearls in a range of vivid colours including gold, peacock, green and chocolate, and although they only have a small yearly output, they command premium prices. 

Freshwater Pearls

Freshwater pearls are grown in rivers and ponds using mussels, and many pearls can be grown within a single shell in a much shorter time that saltwater counterparts. Although freshwater pearls can occur in various colours, white, pink and pastel shades are by far the most common.

There are two ways in which freshwater pearls can be cultured; they can be tissue-nucleated, which means that a small piece of foreign mantle is grafted into the host. This process will usually result in a baroque-shaped pearl.

10-11mm Freshwater pearls, 0.04ct of diamonds, 18ct yellow gold clasp. Part of the Yoko London Classic Collection. Image courtesy of Yoko London.

Cultured freshwater pearls can also be bead-nucleated in the same way as saltwater pearls. Shapes range from round to unique baroques with names like fireball, soufflé and Edison.  However, other shapes are possible, including star, coin and heart. Colour, lustre and size are the most important value factors but they tend to be less lustrous than Akoya pearls.

Considering Value

When considering purchasing freshwater and saltwater pearls, whether natural or cultured, there are five key aesthetic features to take into account in order to make a good investment:

Pearl size

The general rule is the bigger the pearl, the higher the value.

Pearl lustre and orient (shine)

Lustre is a key factor for the majority of cultured pearls and it relates to the amount of light return from the surface of a pearl. Orient is the iridescence of the pearl, a subtle feature created by dispersion of light between platy crystals of aragonite creating different tones.

Pearl colour (shade)

White is a timeless favourite but the value of particular colours is driven by what is popular in fashion at any given time.

Pearl shape

A perfectly spherical shape is always most valuable for pearls.

Pearl surface condition

A near perfect surface condition free from blemishes such as bumps, dimples, scratches and spots will significantly raise the value and market desirability of a freshwater or saltwater pearl.

Read more: Radiocarbon Age Dating on Natural Pearls

While it would always be a very special and rare treat to be able to purchase a set of lustrous, natural round pearls, high quality cultured pearls can make equally, if not more, stunning jewels and a very good investment, particularly when mounted in fine settings from luxury jewellers; exquisite pieces from Harry Winston, Van Cleef & Arpels and Buccellati have recently fetched tens of thousands dollars at auction.

In spite of such awesome prices, the great thing about pearls is that they can be one of the more affordable gemstones to purchase and seem to perennially exude glamour and luxury. If you look out for quality, size, lustre, colour, shape and surface, you are sure to find a fabulous and timeless jewel.

Interested in developing your knowledge about gemstones? Have a look at our upcoming workshops.

Cover Image: 12-13mm South Sea pearls, 0.38ct of diamonds, set in 18ct white gold. Part of the Yoko London Classic Collection. All images courtesy of Yoko London: 

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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.

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