Coloured Gemstones from Brazil: Past, Present and Future

Guy Lalous ACAM EG presents us a historical review, and an updated overview, of the Brazilian coloured stone industry. His latest Journal Digest from Gem-A’s Journal of Gemmology (Winter 2017. 35.8) also examines the effects of China’s emergence as a consumer market on Brazil’s gem industry.

Recent information on Brazilian gem occurrences is presented from data compiled by the Brazilian National Department of Mineral Production (DNPM) and other resources.

This suite of morganite (16.69–22.22 ct) was cut from Brazilian rough
Figure 1: This suite of morganite (16.69–22.22 ct) was cut from Brazilian rough. The gems were fashioned into various custom cuts, including (from upper left to lower right) Concave, Super Trillion, Deep Concave and StarBrite. Courtesy of John Dyer; composite photo by Ozzie Campos.

The history of Brazilian gemstones began with the early colonisation period. No gem deposits were found until 1573, when emeralds were discovered in the present-day Governador Valadares area. In the late 17th century, the legendary gold mines of Sabarabussu were discovered, located to the east of the present city of Belo Horizonte.

The ensuing rush then expanded north, unearthing the first diamond deposits a few decades later near the town that would become Diamantina. The exploration for gold and diamonds overshadowed other gem mining activities until the 19th century, apart from the Imperial topaz deposits found nearby in the old Minas Gerais capital of Ouro Preto. It was during this time that the term ‘garimpeiro’, which refers to independent miners, emerged.

Germans originating from Idar-Oberstein immigrated to the present-day state of Rio Grande do Sul, and to the north-eastern part of Minas Gerais, after Brazil’s independence from Portugal in 1822. During this period, Idar-Oberstein was experiencing economic troubles as the production from its agate mines dwindled.

The immigrants brought their cutting and polishing techniques with them, and initiated the first stage of development for the modern Brazilian coloured stone industry.

Among the most notable early finds was the 110.5 kg Papamel aquamarine, discovered in 1910 in the Marambaia Valley, as well as numerous emerald deposits. On the eve of World War II, gem-related activities in Brazil were mainly limited to mining, while the end of the war saw the widespread closure of industrial mineral mines in Minas Gerais. Many of these mines turned exclusively to gem production and, accordingly, local lapidary and other trade activities grew dramatically. The latter half of the 20th century was also marked by the emergence of a Brazilian jewellery industry.

What about Brazilian Emeralds?

Emerald is the green variety of the mineral beryl, coloured by chromium and vanadium.  These trace elements are normally concentrated in quite different parts of the Earth’s crust, but complicated geological processes enable these contrasting elements to find each other, and crystallise into one of the world’s most beautiful gemstones.

Geological Processes of Brazilian Emeralds

Brazilian emerald mineralization belongs to the classic biotite-schist deposit, which was formed by the reaction of pegmatitic veins within ultrabasic rocks. The granitic pegmatite injection makes the emerald crystallise at the contact zone between these chemically contrasted rocks.  The pegmatite brings beryllium, while the ultrabasic rock contains chromium and vanadium, and the reaction between them is called metasomatism.  This reaction is, however, only possible when geological fluids are present enough to ensure the transportation of the elements.  Pegmatite-free emerald deposits, linked to ductile shear zones, are also known in Brazil (G. Giuliani).

Weighing 6.11 cm, this emerald is unusually large for clean material from Brazil
Figure 2: Weighing 6.11 ct, this emerald is unusually large for clean material from Brazil. Courtesy of Paul Wild; photo by Jordan Wilkins.

Brazilian emerald production occurs in three states: Goiás, Bahia, Minas Gerais.  The first emerald site in modern times was discovered in 1912 in the deep south of Bahia State, near Brumado. Other emerald discoveries followed in the early 20th century at Ferros, in Minas Gerais; at Itaberaí, in  Goiás and at Anagé in Bahia.

More recent discoveries include: Itabira (1977) and Nova Era (1988) in Minas Gerais; at Santa Terez­inha de Goiás (1981) in Goiás; and in the Serra de Carnaíba region (1983) of Bahia.  Since the 2000s, Brazil has been ranked third among the world’s leading emerald producers, behind Colombia and Zambia.

Paraiba Tourmaline from Brazil

Paraíba tourmalines represented the first recorded instance of copper occurring as a colouring agent in this mineral. The blue-to green colours are primarily due to copper (Cu2+).

Figure 3: The bright coloration shown by Paraíba tourmaline (here, 5.73 ct) as well as its rarity have contributed to its high value. Photo and stone courtesy of Paul Wild.

Paraíba tourmaline—for its singularity and its profitability— became perhaps the most important discovery of the 20th century. Uncovered for the first time in 1987, in the São José da Batalha district of Paraíba, this tourmaline’s unique ‘neon’ blue-to-green coloration contributed to making it one of the most valuable coloured stones on the global market. Production of Paraíba tourmaline has, unfortunately, dwindled in recent years.

Global Gemstone Trade - The Role of Brazil 

In the late 1990s, Brazilian gems were mainly exported to the United States, Europe and Japan, with gem exports to Hong Kong and India increasing too. Finally, another major player emerged: China. The increase in exports of rough gem material to Asia has become very significant and, for a time, most of it was cut and polished in India and China.

Chinese brokers began to arrive in the quartz-producing areas at the beginning of the 2000s. These brokers bought most of the cheaper gems, contributing heavily to a rise in mining activities. Over the years, Chinese dealers acquired a greater variety of stones, showing a preference for tourmaline, especially rubellite.

One consequence of the increasing trade with China, though, has been the collapse of Brazil’s domestic gemstone cutting and polishing industry for low-value materials. The negative effect on local economies is particularly apparent in north-eastern Minas Gerais. In Teófilo Otoni, of the 2,700 lapidary businesses operating in 1993 only 360 remained in 2005. In 2011, China was the main partner in Brazil’s gem commerce, with 2013 seeing Brazil export 60% by weight, or 25% by value, of its output to China (up to 50% by value if Hong Kong is included).

Brazil, however, still remains significant for cutting higher-value gemstones. Teófilo Otoni and Governador Valadares remain the major gem cutting and trading centres. Various urban centres host the country’s main gem fairs, which are small venues compared with other international shows.

An important factor to note is the informal trading of gems; several hundred individuals travel around the country and acquire rough material to sell later in the metropolitan areas. The widespread adoption of modern technology, such as the internet and digital cameras, has revolutionised traditional methods of trading over the past decade, making middlemen somewhat obsolete.

Other factors have also diminished Brazilian gem production, such as the evolution of the international market. Many African countries now produce a larger variety and quantity of gems than they did a couple of decades ago, and those stones are usually sold at lower prices than those from Brazil. The high cost of mining is another reason for the recent decline in mining and production. According to the DNPM, there were 2,294 gem occurrences in 401 different municipalities as of 2013, with 49.3% of them in Minas Gerais and 19% in Rio Grande do Sul.

What about Santa Maria Aquamarine?

One of the rarest and most expensive varieties of aquamarine, the "Santa Maria", has a deeply saturated blue colour, with no hint of green or yellow. Named in honour of Santa Maria de Itabira where these stones were first discovered, the original deposit is now almost depleted, and today most of the Santa Maria colours are also found in several sub-Saharan countries of Africa, including Mozambique.

The colouring process is due to the Fe2+ - Fe3+ charge transfer. This feature is associated with intense absorption and strong pleochroism, and this process cannot be induced by heat treatment. The stones are nearly colourless in the direction of the optic axis, and intense dark blue perpendicular to the optic axis.

Figure 4: This 22.93 ct aquamarine is from Santa Maria in Minas Gerais. Courtesy of Paul Wild; photo by Jordan Wilkins.

Quartz and Topaz from Brazil 

Quartzes are abundant in Brazil. Stunning crystal clusters, appreciated by collectors, come from hydrothermal veins found in the Curvelo and Corinto areas of Minas Gerais. Rose quartz is found in pegmatites in the north-eastern part of Minas Gerais, while rutilated quartz has been produced mainly in the Novo Horizonte District of Bahia State.

Huge amethyst geodes, and most of the agates of the country, have been mined in abundance in Rio Grande do Sul. Citrine is mined there as well, but most of the citrine produced and exported from Brazil is actually heat-treated amethyst.

Figure 5: This large colour-zoned tourmaline from Brazil weighs 144.75 ct. Courtesy of Paul Wild; photo by Jordan Wilkins.

The country accounts for much of the world’s topaz production. Many stones are colourless, or tinged very light blue, and laboratory irradiation creates the bright blue colour. A post irradiation annealing is performed by a heat treatment of the topaz for several hours at around 200°C. The production of the rare and famous orange to orange-pink Imperial topaz has, however, greatly diminished, and good-quality material is particularly scarce on the local market.

Figure 6: The colour of this Swiss Blue topaz (13.73 ct, StarBrite cut) was produced by laboratory irradiation, using colourless or pale blue starting material from Brazil. Courtesy of John Dyer; photo by Ozzie Campos.

More Gemstones of Brazil 

Beryl production is dominated by light to medium-dark blue aquamarine, of which Brazil may be one of the largest exporters, with other beryls including heliodor and morganite. Most production of chrysoberyl is still located in Minas Gerais and Bahiam, but good quality Alexandrite is now harder to find locally and only small quantities are available. Collectively, Emeralds and other beryls, tourmaline, topaz and all types of quartz are the most widely mined coloured stone resources nationwide.

What's Next for the Brazilian Gemstone Industry?

Improved living conditions in rural areas have led to an evolution of the workforce, which is becoming less interested in, and less reliant upon, mining via illicit operations. Brazilian gem exports remain robust and gem mining is seeing a shift toward bigger and more professional companies.

The quantity of mining areas remains substantial and, probably, to a large extent underexplored. Yet, the future of the Brazilian gem industry may depend more on social issues, global market trends and an ability to establish efficient trade relationships.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled '‘Coloured Stone Mining and Trade in Brazil: A Brief History and Current Status’ by Aurélien Reys 2017/Volume 35/ No. 8 pp. 708-726

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Characterising Mexican Amber From the Yi Kwan Tsang Collection

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 delves into a feature on Mexican amber and the use of FTIR spectroscopy to determine provenance from the Winter 2017 issue. 

Amber is an organic gem. Organic gems are the products of living, or once-living, organisms and biological processes. Amber derives from fossilised resins produced by prehistoric trees.

Such resins are produced by plants in response to certain circumstances, such as defence against insect pests or protection of wounds.The most important resin-producing plant families are classified among the gymnosperms (conifers) and the angiosperms (flowering plants).

The fossilisation process of amber involves a progressive oxidation, where the original organic compounds gains oxygen, and polymerisation, which is an additional reaction where two or more molecules join together. This process produces oxygenated hydrocarbons, which are organic compounds made of oxygen, carbon and hydrogen atoms.  A peculiarity of amber is that it may perfectly preserve an organism in its original life position. 

What about the Yi Kwan Tsang Collection?

This collection consists of 115 amber samples from Chiapas, many of which contain abundant plant and animal fossil inclusions. The ambers were acquired over a 10-year period (~2004–2013) in San Cristóbal de las Casas, Mexico. The collection was displayed in 2015 at the Tucson Gem and Mineral Show and in 2016 at the Beijing International Jewellery Fair. 

Figure 1: This necklace from the Yi Kwan Tsang Collection contains amber beads from the San Cristóbal de las Casas area of Chiapas, Mexico, and was made by Francesca Montanelli (Lutezia Jewels, Stradella, Italy). Photo by Francesca Montanelli.

This article characterises Mexican amber from the Yi Kwan Tsang Collection using a variety of methods, including those not very common in gemmology, such as taxonomy studies and mass spectrometry, using techniques optimised for organic molecules. Some of the data was compared to those obtained from amber samples from the Baltic Sea and the Dominican Republic. 

The most important amber mines in Mexico are located in the area of Simojovel, but there are several deposits elsewhere in Chiapas State. They date back to the Late Oligocene/Early Miocene (23-13 Ma).

The area is characterised by three stratigraphic units that contain amber. From bottom to top, these are the La Quinta Formation (28–20 Ma), the Mazantic Shale (23–14 Ma) and the Balumtum Sandstone (16–12 Ma). Most of the amber deposits are associated with lignites, friable shales and deltaic clays in the sandstone. 

There are hundreds of amber mines in the tropical forest around Simojovel, and the amber is mined manually using hammers and chisels. 

The twenty-seven samples examined for inclusions were transparent and typically ranged from golden yellow, orange and orange-red to dark orange; a few pieces were dark brown, and some displayed a little natural green colouration. Internal features consisted of inclusions, as well as less common colour variations and small surface fractures. The fractures are probably related to stress associated with the polymerisation of the resin. 

All of the pieces were inert to short-wave UV radiation but displayed weak to strong fluorescence to long-wave UV. The RI values were constant (1.540), and they were similar to those of Baltic and Dominican amber. Average SG values were found to be homogeneous and relatively low (1.03). 

What about taxonomic classification, taxa and phylum?

Taxonomic classification is a hierarchical system used for classifying organisms to the species level. A taxa is a group in a biological classification, in which related organisms are classified. Phylum is a taxonomic ranking that comes third in the hierarchy of classification, after domain and kingdom.  Organisms in a phylum share a set of characteristics that distinguishes them from organisms in another phylum.   

What about arthropods?

The largest phylum of creatures on Earth without a doubt is Arthropoda, both in terms of number of species and in total number of individuals. There are nearly 1 million species of Arthropods, with over 90% of them being insects. 

The determination of the taxa of the botanical and animal inclusions was difficult because the species that lived in Chiapas during the Oligocene-Miocene were different from the modern ones.

The most important plant inclusions were represented by a petal and a leaflet of the genus Hymenaea, more precisely the species Hymenaea mexicana, which is now extinct. Animal inclusions were more common. They consisted of arthropods such as winged termites and a planthopper. Isolated termite wings were detected as well.

The presence of isolated termite wings is extremely rare and the planthopper species, Nogodina chiapaneca, has only been found in amber samples from Chiapas. It is an extinct species dated to the Middle Miocene, and it lived in a tropical or subtropical climate. The presence of an arthropod of the genus Ochlerotatus (female mosquito) also indicates an aquatic environment.  

Figure 8aFigure 2: (TOP) A flower petal inclusion of the species Hymenaea mexicana (Fabaceae family, Late Oligocene–Early Miocene; Poinar and Brown, 2002 and Calvillo-Canadell et al., 2010) is seen in this Mexican amber sample. The petal measures 1.1 cm long and 0.7 cm wide, and shows a narrow midrib base and basal laminar lobes with a central vein and branches of secondary veins. It appears completely glabrous (smooth). (MAIN ARTICLE IMAGE) The H. mexicana leaflet in this Mexican amber measures 3.3 cm long and 1.1 cm wide. The surface is glabrous and its veins are not visible in this view. Photomicrographs by V. L. Villani. 

FIGURE 11aFigure 3: A planthopper of the species Nogodina chiapaneca (order: Hemiptera, family: Nogodinidae; Solórzano Kraemer and Petrulevicius, 2007) is shown at the bottom of this sample. It measures 11 mm long, and displays a rounded head and a clearly visible thorax with one foreleg. The wings have several veins, but the scales are not preserved. This species is known only from Chiapas amber. Photomicrograph by V. L. Villani. 

FIGURE 12Figure 4: These winged termites and isolated wings (rare in Mexican amber) of the order Isoptera (reclassified as part of Blattodea) were likely trapped at the beginning of the wet season, when termites start to swarm and then shed their wings. The length of the wings is ~1.1 cm. Photo by V. L. Villani. 

What about X-ray powder diffraction?

X-ray powder diffraction (XRD), is an instrumental technique that is used to identify minerals, as well as other crystalline materials.The method is based on the scattering of x-rays by the crystals. X-rays are diffracted by each mineral differently, depending on what atoms make up the crystal lattice and how these atoms are arranged. An X-ray scan provides a unique "fingerprint" of the mineral.

Natural resin/amber is amorphous, so XRD analysis does not yield information on the amber itself but can identify mineral inclusions. XRD identified very small amounts of refikite and hartite, as well as calcite.  Calcite was identified by a diagnostic peak at 3.03 Å (or 29.8° 2θ) in the amber from Chiapas only.  Refikite and hartite have a composition similar to that of resin, but possess a crystalline structure.  They are probably associated with the polymerisation process of the resins. 

What about amber classification?

Amber can be classified according to two criteria: their place of origin, and their chemical composition. When succinic acid is present the amber is classified as succinate, when succinic acid is lacking it is considered as a resinite. 

Mass spectrometry was performed to deter­mine the presence of free succinic acid in the amber samples. Confirmation of succinic acid is obtained from the m/z 117 ion (the negative ion mass peak) corresponding to (M-OH)– of succinic acid. The mass spectrum of the Mexican amber did not show the m/z 117 ion, so the level of succinic acid in this amber was lower than the limit of quantisation (1 ppm by weight), classifying it as a resinite.

The Dominican sample showed a spectrum very similar to that of the Mexican amber, indicating the absence of succinic acid, while the m/z 117 ion was clearly identifiable in the spectrum of the Baltic amber sample.

What about Infra-red spectroscopy and the “Baltic Shoulder” in Baltic amber?

Infra-red spectroscopy is the most effective scientific method for identifying fossil resins.  With this technique, broad absorptions will be witnessed in Baltic amber in the 1260-1160 cm-1 range.  Those are assigned to C-O stretching vibration.  These features known as “Baltic Shoulder” are specific to Baltic amber and are related to the presence of succinic acid. 

Three wavenumber ranges that are important for amber characterisation are 3700–2000 cm–1, 1820–1350 cm–1 and 1250–1045 cm–1; these regions are associated with hydroxyl and carbonyl groups and to C=C double bonds. 

FIGURE 15Figure 15: FTIR spectroscopy of a Mexican (Chiapas) amber shows typical peaks at: 3600–3100 cm–1 (broad absorption band due to the O-H stretching vibration); 2965 and 2860 cm–1 (C-H stretching); 1740 cm–1 (C=O stretching, esters and acid groups); 1450 and 1380 cm–1 (C-H aliphatic hydrocarbons); 1260–1030 cm-1 (C-O stretching aromatic esters and secondary alcohols); and 846 cm–1 (C-C stretching of unsaturated olefins). 

The fossil inclusions observed in Chiapas amber in this study are consistent with a sub-tropical forest and FTIR spectroscopy was confirmed as a useful technique to determine the provenance of the amber samples.   

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Characterization of Mexican Amber from the Yi Kwan Tsang Collection‘ by Vittoria L. Villani, Franca Caucia, Luigi Marinoni, Alberto Leone, Maura Brusoni, Riccardo Groppali, Federica Corana, Elena Ferrari and Cinzia Galli 2017/Volume 35/ No. 8 pp. 752-765 

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幸運なことに1月に生まれた方は、美しく、明るく、鮮やかなガーネットが誕生石です。虹色 の宝石、ガーネットは実に様々な色をもつ鉱物で、業界では処理を行わないごくわずかな宝石 の一つに数えられます。

ガーネットは様々な彩度、種類、色相がある絵具のパレットのようであり、豊かな歴史と伝説 があることでも知られています。ヴィクトリア時代の王族、古代エジプト時代の歴代王、古代 ローマ人をはじめ、多くの時代で好まれた宝石です。

きれいなホーステール・インクルージョンが見られるデマントイド・ガーネット。写真:Pat Daly


ガーネットは赤色部に強いスペクトルもち、ざくろの実を思い起こさせるような赤褐色を帯び たオレンジ、濃いバイオレット、ロイヤル・レッドの色合いを示します。「ガーネット (garnet)」は14世紀中期に使用されていた濃い赤色を意味する英語の「gernet」にちなみ、 さらにはラテン語の「ざくろの実(種)」を意味する「グラタナス(granatum)」に由来する 名前です。これはガーネットの結晶がざくろの種や濃い赤色に似ているためです。ガーネット とは、赤色のパイロープ・ガーネットから趣のある緑色をしたツァボライト・ガーネットまで 、鉱物のグループを指す用語です。

ガーネットの色が様々であるのは、マンガン、鉄、クロムなどの微量金属によるものです。ガ ーネットの変種で異なる化学組成は、硬度に6.5-7.5の範囲をもたらす原因となります。

偏光器下でのガーネットとペリドタイト。写真:Pat Daly

左: フェザー・インクルージョンを伴うスペサルティン・ガーネット。右:結晶インクルージ ョンを伴うヘソナイト・ガーネット。写真:Pat Daly


ガーネットには、同じ結晶構造にもかかわらず、若干異なる化学組成をもつ5つの主な変種があ ります。

  • アルマンディン:紫色から帯橙赤色
  • パイロープ:紫色から帯橙赤色
  • スペサルティン:橙色から黄色
  • アンドラダイト:黄色から帯黄緑色
  • グロッシュラー:無色から黄色、帯橙赤色、鮮やかな緑色(稀少)



ガーネットはオーストラリア、インド、チェコ共和国、ミャンマー、ブラジル、スリランカな ど世界中の多くの国々で見つかります。19世紀にはジュエラーのピーター・カール・ファベル ジェ(Peter Carl Fabergé)が製作した多くの作品にガーネットが用いられ、ロシアの王族に愛 されました。ボヘミアやロシアは19世紀の最初のガーネットの産地として記録されていますが 、現在では、タンザニアやナミビアでガーネットが豊富に採取されます。

デマントイド・ガーネットのホーステール・インクルージョンを拡大した写真。写真:Pat Daly


ガーネットには長い間語り継がれてきた歴史と多くの言い伝えがあります。「誠実の宝石」と して知られるガーネットは健康、繁栄、平和の力をもち、この石を身に着ける者は善い行いを して報われると言い伝えられてきました。鮮やかで強い印象をもつガーネットは、活気に満ち た1月に生まれの方に相応しい石であると考えられています。そして、ガーネットを着ける 人は新たな目標をもち、希望と幸福を感じながら新年を迎えることができるとされています 。

また、戦いに臨む人々は勝利と身の安全を祈る護符としてこの宝石を使いました。多くの戦士 たちが病気や貧窮を防ぐために身に着け、戦いで負った傷に石を当て、治癒を促すものとして 使用しました。さらに歴史を遡ると、エジプトではガーネットを生命の象徴としてジュエリー や彫刻に象嵌しました。古代ローマ人は好んでガーネットをシグネット・リング(印章付きの 指輪)に用いました。これは石に沈み彫りを施し(インタリオ)、重要な文書に封印をするも のです。健康にも効果があるとされたため、歴史を通じて聖職者や貴族階級の間で珍重され続 けました。

マリ産のガーネット。写真:Pat Daly

今日、ガーネットはリング、ペンダント、ティアラなど様々なジュエリーに使用されています 。その色と透明度が、作品を価値ある素晴らしいものに仕上げているのです。

ガーネットが使われた大変有名なジュエリーの例として、ヴィクトリア時代に製作されたスミ ソニアン・パイロープの髪飾り(the Smithsonian Pyrope Hair Comb)が挙げられます。頂上の 中央石を取り囲み、ティアラの形をした髪飾りを装飾するパイロープ・ガーネットは、ボヘミ アの鉱山(現在のチェコ共和国の一部)で採取されたものです。このローズ・カットを施した ガーネットはメッキをした金属またはイエロー・ゴールドにセットされました。この作品が流 行したヴィクトリア時代には人気のあるスタイルでした。

今年の成功を祈る護符として、また1月生まれの方への贈り物として、個性的なガーネットのフ ァイアと煌めきは、強く感情を揺さぶり、時を超えた美しさをもつことでしょう。

 針状のルチル・インクルージョンを伴うマラヤ・ガーネット。写真:Pat Daly

表紙:結晶インクルージョンを伴うアルマンディン・ガーネット。写真:Pat Daly ©Gem-A

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オパールの価値をどう評価するか? 初心者のための価格ガイド


かつて、オパールは「縁起が悪い」ことで知られていましたが、近年はジュエリーの中でも需要の多い宝石の1つとなりました。しかし、あるオパールが数千ポンドの価値がある一方で、他のオパールはほんの数ポンドあるいは数ペンスにしかならないのはなぜでしょうか?ここでは、Gem-A Instrumentsのマネージャー、サマンサ・ロイド FGA EGが、オパールの価値を決める要因と、素晴らしい石と平均的な石とを識別するための簡単かつ必須な情報を提供します。







オパール本体が透明の場合 ―ライト・クリスタル・オパールとも呼ばれる― 色斑は表面より下の方に見られます。このような石は非常に高い価格になります。暗い地色をもつ ―時に色の輝きを強める― ブラックあるいはダーク・オパールもまたよく知られています。これは、最も稀少で最も価値あるオパールの変種です。










エチオピア VS オーストラリア 


重要なのは、この地域で産出したオパールは含水量が低いことです。つまり、乾燥の影響を受けにくく「ひび割れ」―石の耐久性に影響を与えるヘアライン・フラクチャー(細い線の割れ)― を起こしにくいことを意味します。



エチオピア、オーストラリア・オパールの写真撮影は、ハットンガーデンにある宝石のサプライヤー、 Marcus McCallum FGA, のご協力に感謝いたします。

オパールについてさらに知りたい方は、Gem-Aの図書室にAndrew and Damien Cody著「The Opal Story」をお勧めします。

Gem-A会員の方は、ウェブサイトにアクセスしGems&Jewellery Spring 2017 / Volume 26 / No. 1 にて記事全文をご覧いただけます。

表紙はオーストラリア・オパールのサンプル。すべての写真提供:Marcus McCallum.

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