Gems & Gemology

Spring 2018 G&G: Color Origin of Green Diamonds, Iridescence in Hematite, and the Outlook from Tucson


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Colored by atomic-level defect arrangments, natural-color green diamonds are among the rarest gemstones found on Earth. The cover photo showcases diamonds that display a range of green hues. The diamonds in the rings (clockwise from top) are a 4.17 ct Fancy Vivid yellowish green, a 10.18 ct Fancy Intense yellow-green, and a 3.88 ct Fancy Vivid green, respectively. The unmounted diamonds in the top row are a 1.42 ct Fancy Vivid bluish green, a 1.01 ct Fancy Vivid yellowish green, and a 2.06 ct Fancy Vivid green-blue. In the bottom row are a 0.55 ct Fancy Vivid green and a 0.55 ct Fancy Vivid bluish green. Photo by Robert Weldon/GIA, courtesy of Optimum Diamonds.

Green is among the rarest hues found in colored diamonds, and there have been few published studies. The lead article of the Spring 2018 Gems & Gemology reviews the characteristics of these uncommon gems as observed at GIA’s labs. Other articles explore the causes of iridescence in Brazilian hematite and the application of DNA techniques to identifying Japanese akoya cultured pearls. The issue also includes coverage of the annual Tucson gem shows, the 2018 Challenge, and the results of the Dr. Edward J. Gübelin Most Valuable Article Award.

NATURAL-COLOR GREEN DIAMONDS: A BEAUTIFUL CONUNDRUM

Ranges of hue in natural-color green diamonds.
Natural-color green diamonds such as these beautiful gems span a range of colors. From left to right: Fancy Vivid green-blue (2.06 ct), Fancy Vivid yellow-green (1.01 ct), Fancy Dark gray greenish yellow (3.08 ct), and Fancy Intense yellow-green (2.01 ct). Each of the four main color-causing mechanisms usually produces slightly different green hues. Photo by Robert Weldon/GIA, courtesy of Optimum Diamonds.

Diamonds with natural color comprise less than 0.4% of all diamonds submitted to GIA. Of these, stones with a pure green hue are among the rarest. In the last decade, GIA labs have examined more than 50,000 natural-color green diamonds. The lead article, by Christopher M. Breeding, Sally Eaton-Magaña, and James E. Shigley, documents their properties and discusses the radiation exposure and atomic-level defects that cause this hue in diamonds, as well as the difficulties of separating natural-color and treated material.

IRIDESCENCE IN METAMORPHIC “RAINBOW” HEMATITE

Rainbow hematite from Brazil’s Andrade mine.
Rainbow hematite from the Andrade mine in João Monlevade, Minas Gerais, Brazil. Photo © Rock Currier, Mindat.org.

Rainbow hematite was first uncovered from Minas Gerais, Brazil, and since then it has been used in award-winning jewelry pieces. Xiayang Lin and her coauthors investigate the cause of iridescence in this material through electron microscopy, atomic force microscopy, and synchrotron X-ray diffraction analysis. They assert that the iridescence arises from a periodic microstructure, consisting of spindle-shaped hematite nanocrystals containing minor Al and P impurities, resulting from arrested crystal growth.

DNA TECHNIQUES APPLIED TO THE IDENTIFICATION OF JAPANESE PINCTADA FUCATA PEARLS

Experimental steps of DNA extraction and purification.

Between November and January of each year, pearls are harvested at Uwajima from akoya shells operated on either six months prior (classified by the Japanese as tonen-mono) or 18 months prior (koshi-mono). A: An Uwajima pearl farmer’s work hut. B–F: Various scenes from harvesting in a shed on one of the floating rafts. B: Oysters are removed from a net on the floor to separate the living mollusks from the dead ones. C: After the shells are opened on a table, the adductor muscles are removed and the other soft tissues are separated. D: The liquefied soft tissues containing the bead-cultured pearls are collected in a blue plastic bucket. E: The remaining organic matter containing smaller “keshi” pearls is poured into a separator to remove the keshi pearls. F: The bead-cultured pearls are placed into a wooden box for cleaning and subsequent inspection. Photos by K. Saruwatari.

One of the challenges of modern pearl identification is the separation of saltwater mollusk species that produce white, cream, or silver nacreous pearls. A team led by Kazuko Saruwatari used DNA techniques on akoya pearl-producing mollusks from Uwajima, Ehime Prefecture, Japan, to see if the mollusk species could be pinpointed. The authors were able to amplify the 16S rRNA gene from the mantle tissues of donor and host pearl oysters as well as from tiny amounts of pearl powder, and successfully identified the Pinctada fucata species.

LAB NOTES

Fracture-filled square modified brilliant cut diamond.
A face-up view of the near-colorless 1.21 ct fracture-filled square modified brilliant cut. Photo by Robison McMurtry.

In this issue, reports from GIA’s labs include an HPHT-processed diamond fraudulently represented as untreated, the analysis of five CVD synthetic diamonds greater than three carats, and a natural blister resulting from the entombment of a pearlfish within a P. maxima shell.

MICRO-WORLD

Close-up of agate-like banding.
Prominent agate-like banding was seen in the opal. Photomicrograph by Nathan Renfro; field of view 8.22 mm.

Entries in G&G’s section on inclusions and photomicrography spotlight an opal with agate-like banding, a natural diamond with etch channels, and a pink tourmaline crystal within a spodumene host

GEM NEWS INTERNATIONAL

2018 AGTA Tucson Show
Although traffic at the AGTA show was lighter than in some previous years, demand was brisk and most traders were satisfied with their 2018 business. Photo by Kevin Schumacher.

In addition to our annual Tucson report, GNI features entries on aquamarine from Pakistan’s Shigar Valley, Concho pearls from Texas, and an irradiated and annealed blue type Ia diamond.

The Spring issue also contains the 2018 Gems & Gemology Challenge quiz. 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 2018 issue. Mail-in cards and online entries for the Challenge must be submitted by Friday, August 10, 2018.

Jennifer-Lynn Archuleta is the editor of Gems & Gemology.