Gems & Gemology

Summer 2018 G&G: Diamonds Across the Spectrum, Byzantine Garnet, and Sapphires from Yogo Gulch

Jennifer-Lynn Archuleta

August 6, 2018

For more than 120 years, Yogo Gulch in central Montana has been one of the world’s most illustrious sapphire sources. In this issue, Nathan Renfro, Aaron Palke, and Richard Berg offer a mining history and gemological characterization of Yogo sapphires. The cover photo shows a suite of rough Yogo sapphires atop samples of the ouachatite dike rock in which they are found. The largest rough sapphire weighs 8.49 ct. Photo by Kevin Schumacher; courtesy of Don Baide (The Gem Gallery, Bozeman, Montana).
For more than 120 years, Yogo Gulch in central Montana has been one of the world’s most illustrious sapphire sources. In this issue, Nathan Renfro, Aaron Palke, and Richard Berg offer a mining history and gemological characterization of Yogo sapphires. The cover photo shows a suite of rough Yogo sapphires atop samples of the ouachatite dike rock in which they are found. The largest rough sapphire weighs 8.49 ct. Photo by Kevin Schumacher; courtesy of Don Baide (The Gem Gallery, Bozeman, Montana).

Thanks to notable diamonds such as the Hope and the Blue Moon of Josephine, there is greater public awareness of blue diamonds than any other fancy color. The lead article of the Summer 2018 Gems & Gemology reviews the characteristics of the more than 15,000 blue, gray, and violet diamonds examined at GIA’s labs over the past decade, while another article investigates the color origin of Fancy Dark brown-black diamonds from Marange, Zimbabwe. Other diamond-related content includes a synthetic diamonds chart and a new column about the value of natural diamonds to our understanding of the Earth. The issue also features a study of Japanese bead-cultured pearls from Lake Kasumigaura, traces the origins of an engraved Early Byzantine almandine garnet, and examines the characteristics of sapphires from Yogo Gulch, Montana.

NATURAL-COLOR BLUE, GRAY, AND VIOLET DIAMONDS: ALLURE OF THE DEEP

Recently discovered and legendary diamonds in the blue/gray/violet color range
A few recently discovered blue diamonds are displayed alongside some legendary gems within the blue/gray/violet color range. Clockwise from upper left: the 30.62 ct Blue Heart diamond, photo by Chip Clark, © Smithsonian Institution; a 0.77 ct Fancy Dark gray-violet shield cut, photo by Robert Weldon/GIA; a 1.23 ct Fancy Dark gray diamond, GIA photo; the 45.52 ct Fancy Deep grayish blue Hope diamond and Fancy Deep blue 31.06 ct Wittelsbach-Graff diamond, photo by Robert Weldon/GIA, courtesy of the Smithsonian Institution.

Almost all fancy-color diamonds in the blue/gray/violet range attribute their hue to four main causes, which include structural defects and the presence of inclusions. The Summer issue’s lead article, by Sally Eaton-Magaña, Christopher M. Breeding, and James E. Shigley, reports on these color-causing mechanisms. The authors delve into the gemological characteristics and spectral properties of the 15,000+ natural, untreated blue/gray/violet diamonds examined at GIA’s labs over the past decade.

BLACK DIAMONDS FROM MARANGE (ZIMBABWE): A RESULT OF NATURAL IRRADIATION AND GRAPHITE INCLUSIONS

DiamondView images and photos of Marange diamonds that show radiation stains within fractures
DiamondView and visible light images of two diamonds that show radiation stains within fractures in the diamond. Non-luminescent regions in the DiamondView images are associated with extensive radiation damage along fractures in the diamonds. Photos and images by Karen Smit.

Graphite-containing diamonds from the alluvial deposits in Marange, Zimbabwe, may be heat treated to yield gem-quality material, but this may create difficulty in distinguishing these specimens from naturally occurring black diamonds. Karen Smit and her coauthors examine the characteristics of a suite of 40 Fancy Dark brown-black diamonds from Marange in order to document the characteristics that would separate them from treated material. The authors uncover various causes of color in these diamonds.

AN EARLY BYZANTINE ENGRAVED ALMANDINE FROM THE GARIBPET DEPOSIT, TELANGANA STATE, INDIA: EVIDENCE FOR GARNET TRADE ALONG THE ANCIENT MARITIME SILK ROAD

Comparison of inclusions in the Early Byzantine engraved gem with garnets from Garibpet
Comparison of inclusions in the Early Byzantine engraved gem (left) with garnets from the Garibpet deposit (right). A and B: Inclusion-rich core and inclusion-poor rim with sillimanite fibers at the boundary. C and D: Irregularly shaped ilmenite. E and F: Subrounded quartz crystals. G and H: Large long-prismatic apatite crystal (a) with characteristic graphite inclusions, along with short-prismatic zircon (z) and monazite (m) crystals. Photomicrographs by H.A. Gilg.

An almandine garnet engraved with a Christian stepped-cross motif popular on Byzantine coinage is the focus of this study by H. Albert Gilg, Karl Schmetzer, and Ulrich Schüssler. Nondestructive analytical methods indicate that the almandine is most likely from India’s Garibpet deposit, giving credence to garnet trade between the eastern coast of India to the Byzantine Empire during the early Middle Ages.

CULTURED PEARLS FROM LAKE KASUMIGAURA: PRODUCTION AND GEMOLOGICAL CHARACTERISTICS

Strand of Chinese freshwater nucleated cultured pearls from Lake Taihu, cultured by the same <I>Hyriopsis</I> hybrid as Kasumiga pearls.
The Chinese freshwater nucleated cultured pearls in this strand from Lake Taihu, also cultured by the Hyriopsis schlegelii × Hyriopsis cumingii hybrid, are 10 mm in diameter and show a color range and luster similar to Kasumiga pearls. Photo by Ahmadjan Abduriyim.

Freshwater pearl culturing began in pre-World War II Japan and reached its peak between 1970 and 1980, only to be overtaken by Chinese production. Current Japanese production at Lake Kasumigaura—Japan’s second-largest lake—is comparatively low, but these cultured pearls have become quite popular in the United States and Europe. Ahmadjan Abduriyim relates the history of Japanese freshwater bead-nucleated pearls, as well as the current state of production and the characteristics of the many-colored specimens.

GEMOLOGICAL CHARACTERIZATION OF SAPPHIRES FROM YOGO GULCH, MONTANA

Suite containing over 36 carats of Yogo sapphire
This suite contains more than 36 carats of Yogo sapphires. Courtesy of RareSource. Photo by Robert Weldon/GIA.

Yogo sapphires are renowned for their clarity and vibrant blue color, and their unique characteristics make them easily recognizable to experienced gemologists. However, mining interests have long found it difficult to maintain long-term activity at this source. In this article, Nathan Renfro, Aaron Palke, and Richard Berg offer a comprehensive look at the history, gemological properties, and trace-element chemistry of the corundum yielded from this important gem deposit in the United States.

SYNTHETIC DIAMONDS CHART

Features of Synthetic Diamonds
The Summer 2018 G&G contains a foldout wall chart showing the characteristics of synthetic diamonds.

Our newest wall chart, coordinated by Sally Eaton-Magaña and Christopher M. Breeding, presents a visual guide to the growth processes and major gemological spectroscopic features of synthetic diamonds grown by the high-pressure, high-temperature (HPHT) and chemical vapor deposition (CVD) methods. Additional resources for further study are also provided.

LAB NOTES

Diagram of assemblage.
Diagram showing the proposed model of the beryl and glass assemblage. Final faceting is shown for simplicity. A: The first glass segment (green) is attached to the beryl core (gray). B: Both the core and the first segment are cut to make a flat surface for the second segment. C: The second segment (teal) is attached. D: The segments are again cut. E: The third segment (blue) is attached. F: The assemblage is shown rotated 180° for clarity. All current pieces are cut to create the surface for the final piece. G: The fourth and final segment (purple) is attached. H: Omitted for simplicity, the assembled pavilion would likely be polished flat to make room for the crown. Lastly, the crown (pink) is attached. The diagram was generated using Onshape CAD software.

GIA’s labs report on an emerald imitation created from a beryl and glass assemblage, one natural melee diamond found among a batch of 1,092 HPHT synthetic melee, and a large pinkish orange CVD synthetic diamond.

DIAMONDS FROM THE DEEP

Ringwoodite in diamond, and diamonds in Earth.
Ringwoodite is the high-pressure form of the mineral olivine (Mg2SiO4) that occurs between 520 and 660 km below the surface of the earth in the transition zone. The first terrestrial observation of this mineral was in a diamond from Juína, Brazil (top left). Water content in this 30 µm inclusion (bottom left) is around 1.4 wt.%, indicating that the global transition zone could contain at least 2.5 times the amount of water as Earth’s oceans. The majority of diamonds come from the cratonic lithosphere (indicated here as yellow diamonds). Superdeep diamonds (indicated here as white and blue diamonds) are rarer and originate from greater depths, often from the transition zone, which extends between 410 and 660 km.

G&G’s newest section highlights the importance of diamond in understanding our planet’s geological formation. This issue’s column focuses on the role diamonds play in understanding the depths at which water is found in the deep earth.

MICRO-WORLD

Mushroom in Mexican Copal
This copal from Mexico contained a remarkably well-preserved eye-visible mushroom inclusion. Photomicrograph by Nathan Renfro; field of view 9.59 mm.

A mushroom encased in copal, tanzanite with whisker-like merelaniite inclusions, and a star spinel with four and six rays are among the specimens featured in our photomicrography and inclusions section.

GEM NEWS INTERNATIONAL

Garnets under daylight-equivalent (left) and incandescent light (right).
Left: Three rough garnets (left to right, stones 2, 3, and 4) and one faceted garnet photographed in an LED light source with 6000K color temperature (a daylight-equivalent light simulator). Right: The same stones photographed in an LED light source with 3100K color temperature (an incandescent light simulator). The 2.705 ct faceted stone measures 8.08 × 7.18 × 5.49 mm. Photos by Kevin Schumacher.

Notes from the around the world highlight color-change grossular garnets from Ethiopia, a Mozambican green-blue Maxixe-type beryl, the Gemstones and Sustainable Development Knowledge Hub, and the separation of glass-filled rubies using the DiamondView instrument.

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