Gems & Gemology

Spring 2017 G&G: Brazil’s Diamond Production, Cuprian Liddicoatite, Silicon in Corundum, and Tucson Coverage

Jennifer-Lynn Archuleta
May 8, 2017
 Spring 2017 Gems & Gemology
The lead article of this issue covers Brazilian diamond production, which reached its peak in the late 1700s. Our cover shows a brooch dating back to the mid-19th century. This piece contains a 0.77 ct natural Fancy Light pink antique pear-shaped diamond set in darkly patinated silver ribbons over gold. The total weight of the diamonds exceeds 5.20 carats. Photo by Robert Weldon/GIA, courtesy of Lang Antique & Estate Jewelry.

From Brazilian diamonds and Japanese jadeite to Indian iridescent quartz and Polish by-product zincite, a world of wide-ranging topics is covered in the Spring 2017 issue of Gems & Gemology. The issue looks at historical and unexpected sources of gemstones, plus surprising discoveries revealed through advanced chemical analyses. News and updates from the annual Tucson gem shows and our multiple-choice Challenge quiz are also included in the Spring issue.

BRAZILIAN DIAMONDS: A HISTORICAL AND RECENT PERSPECTIVE

Badge of the Three Military Orders
The Portuguese Placa dos Três Militares, or Badge of the Three Military Orders, was
made in 1789, and is now part of the crown jewelry collection of the Ajuda National
Palace in Lisbon. The badge consists of Brazilian diamonds as well as emeralds and
rubies, and it is designed in a way to conceal the metal mounting and emphasize the
gems. Photo by Manuel Silveira Ramos, courtesy of the Ajuda National Palace.

Brazil is known for its wealth of colored stones, but it was at one time the world’s leading diamond producer. For three centuries Brazil has supplied a steady, if relatively small, supply of diamonds to the world market. It is also a source of superdeep and fancy-color diamonds and carbonado. In Brazil, diamonds are gathered from alluvial deposits (primarily in Mato Grosso and Minas Gerais), while primary sources remain unknown. Authors Darcy Svisero, James Shigley, and Robert Weldon look at Brazil’s centuries-old diamond production with historical perspective, geological insight, and expectations for future mining.

COPPER-BEARING LIDDICOATITE TOURMALINE

Cuprian liddicoatite tourmalines
Six of the 13 cuprian liddicoatite tourmalines from this study. The faceted stones weigh 1.59–9.63 ct (average 3.95 ct). They are transparent and have greenish blue, green-blue, and green colors. Photos by Masumi Saito.

Copper-bearing (Paraíba) tourmaline is usually classified as part of the elbaite species. Recently, 13 cuprian tourmalines submitted to GIA’s Tokyo lab were analyzed with LA-ICP-MS for origin determination. While the origin of the stones remains unknown, Yusuke Katsurada and Ziyin Sun discuss the surprising results that led them to classify the samples as liddicoatite, rather than elbaite.

THE ROLE OF SILICON IN THE COLOR OF GEM CORUNDUM

Polished Yogo wafer
Paula Crevoshay’s “Yogo Columbine” pendant displays the remarkably consistent blue color of Yogo sapphires: 243 in all, with a total weight of 13.89 carats. The piece also contains 0.59 ct of yellow sapphires and 1.37 ct of diamonds, set in 18K yellow and white gold. Photo by Orasa Weldon.

The chemical interactions of silicon, titanium, and magnesium are known to influence the color of gem-quality corundum, but until recently the role of silicon has not been fully understood. John Emmett, Jennifer Stone-Sundberg, Yunbin Guan, and Ziyin Sun explore how ion implant standards and the application of SIMS has led to accurate measurement of silicon in corundum for the first time. This measurement makes clear the important role of silicon in corundum color; the blue of Yogo sapphire from Montana is used to illustrate this discovery.

JAPANESE JADEITE: HISTORY, CHARACTERISTICS, AND COMPARISON WITH OTHER SOURCES

Jadeite boulder
A large, attractive jadeite boulder from Itoigawa, Japan, characterized by mixed white
and green colors. This boulder weighs 40.5 kg and measures approximately 39 cm
high, 32 cm long, and 26 cm wide. Photo by Ahmadjan Abduriyim.

Gem-quality jadeite jade has been known from Japan for millennia. In this article, Ahmadjan Abduriyim, Kazuko Saruwatari, and Yusuke Katsurada examine the historical context, chemical composition, and color of Japanese jadeite. They also compare characteristics of samples from the Itoigawa, Omi, and Wakasa regions to jadeite from Burmese, Guatemalan, and Russian deposits.

CAUSES OF IRIDESCENCE IN NATURAL QUARTZ

Iris quartz
Iridescence is clearly visible on the minor z face of this iris quartz from Jalgaon, India.
Photomicrograph by Peter J. Heaney; field of view 4.5 mm.

The phenomenon of iridescence in gemstones is caused by either thin films or repetitive substructures. In an exploration of iris quartz specimens from Jalgaon District, India, Xiayang Lin and Peter Heaney discover a ridge and valley structure, produced by differential etching on the material’s minor z face, which produces its iridescent effect.

CHARACTERISTICS OF GEM-QUALITY BY-PRODUCT SYNTHETIC ZINCITE

Synthetic zincites
Most of the by-product synthetic zincites in this study had a reddish orange to yellow color; one sample (ZnO-G) was yellowish green. Top row, left to right: samples ZnO-LC (69.23 ct) and ZnO-SC (31.71 ct). Bottom row, left to right: samples ZnO-G (0.32 ct) and zin-7 (5.61 ct). Composite photo by Ji Zhang.

Synthetic zincite has been found in industrial kilns in Poland as a by-product of zinc oxide powder production. This accidental zincite has better transparency and brighter color than its natural counterpart. Ji Zhang, Yujie Gao, and Guanghai Shi examine samples of by-product synthetic zincite from a Polish foundry to establish their distinguishing characteristics, identify their inclusions, and find their cause of color.

LAB NOTES

Green synthetic diamond
This 0.42 ct green HPHT synthetic diamond displayed strong growth sectors that were observed without magnification. Photo by Jian Xin (Jae) Liao.

GIA’s gemologists report on notable lab submissions, including a surprising source of green color in HPHT synthetic diamond, a Paraíba tourmaline with an unusual submetallic coating, and a cape diamond showing yellow phosphorescence.

MICRO-WORLD

Photomicrograph
In this scene, pyrite inclusions have dissolved, leaving behind hematite and goethite, both of which react dramatically to polarized light. Photomicrograph by John Koivula; field of view 10 mm.

The inner world of gemstones continues to captivate, and the Spring 2017 section features sapphires showing negative crystals, flame-like structures in precious opal from Ethiopia, and a mysterious inclusion combination in Sri Lankan topaz.

GEM NEWS INTERNATIONAL

28.21 ct freeform Oregon sunstone
A 28.21 ct freeform Oregon sunstone from the Dust Devil mine is the centerpiece of this ring. Photo courtesy of Sonja Kreis Unique Jewelry.

The GNI section contains G&G’s annual coverage of the Tucson gem shows. Highlights include emeralds from a new deposit in Ethiopia and a Burmese gem update following the lifting of U.S. sanctions on Myanmar. Other entries report on the chemical composition of color-change pyrope-spessartine garnet, and a calcite sphere exhibiting strong phosphorescence.

The Spring issue also contains the 2017 Gems & Gemology Challenge. Score 75% or better and you’ll receive a certificate of completion (PDF file); earn a perfect score and your name will be listed in the Fall 2017 issue. Mail-in cards and online entries for the Challenge must be submitted by Friday, August 11, 2017.

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