Gems & Gemology

Winter 2017 G&G: Cullinan-Like Superdeep Diamonds, the Artistic Exquisite Gem Carvings of the Dreher Family, and Matrix-Matched Corundum Standards

Jennifer-Lynn Archuleta

January 22, 2018

Winter 2017 Gems & Gemology
The rough diamond shown on the cover, the 812 ct Constellation, was recovered from the Karowe mine in Botswana in November 2015. It is currently the sixth-largest gem-quality rough diamond ever recorded. It was sold in May 2016 for $63.1 million, the highest price paid for a rough diamond to date. The size, near absence of inclusions, low nitrogen content, shape, and surface texture place it in a special variety known as CLIPPIR diamonds, which are reviewed in the lead article in this issue. Diamonds such as the Constellation, which grow in a geologically distinct way, convey valuable information about the earth’s deep mantle. Photo by Jian Xin (Jae) Liao.

From diamonds that originate deep within the earth’s mantle to pearls cultured along Australia’s eastern coastline, the Winter 2017 issue of Gems & Gemology reports on the many facets of the gemological world. Articles include a look at the origin of Cullinan-like “superdeep” diamonds, the lifelike gem carvings of Gerd and Patrick Dreher, and the characteristics of Australian akoya cultured pearls. The Winter issue also introduces GIA’s new calibration standards sets  for analyzing the chemistry of corundum, compares two different types of detectors in microradiographic imaging of pearls, and features a chart showing inclusions in natural, synthetic, and treated ruby.

THE VERY DEEP ORIGIN OF THE WORLD’S BIGGEST DIAMONDS

Rough CLIPPIR diamonds
Rough CLIPPIR diamonds have physical features akin to the historic Cullinan diamond, which forms the “C” of the CLIPPIR acronym. The size distribution for this variety of diamond is skewed toward large sizes, with the examples shown here ranging from 14 to 91 carats. In rough form, CLIPPIR diamonds are irregularly shaped rather than well-formed crystals and sometimes appear to be broken fragments of once larger diamonds. Photo by Robert Weldon/GIA; courtesy of Gem Diamonds Ltd.

The lead article of the Winter 2017 issue reviews the remarkable diamonds found deep in the earth’s convecting mantle. In a follow-up to his December 2016 article in Science magazine, Evan Smith and coauthors examine 83 of these large, pure, Cullinan-like diamonds, known for their high clarity and D color grades. The samples used in this paper, along with their inclusions and other properties, provide a glimpse into the earth’s geologic formation.

GEM VIRTUOSOS: THE DREHERS AND THEIR EXTRAORDINARY CARVINGS

Gerd Dreher agate lily and Patrick Dreher citrine mouse
This Gerd Dreher lily carving (2005) is from a single piece of Brazilian agate and measures 11.0 cm in length. His skills are on exhibit through his use of the material and his technique, such as the curled petals that demonstrate a wealth of detail even on their undersides. The leaf, stamen, and enameled pistil accents are crafted from 18K gold and set with diamonds. Patrick Dreher’s citrine mouse (2015), 6.0 cm in length, shows attention to the animal’s stance and demeanor as well as fine detail in the carving of the fur. Photo by Robert Weldon/GIA, courtesy of the William F. Larson family.

The German town of Idar-Oberstein is known for its gem cutting and carving industry. Coauthors Robert Weldon, Cathleen Jonathan, and Rose Tozer explore the town’s rich history and the carving tradition of the Dreher family, which has passed down its skills through thirteen generations. The work of contemporary masters Gerd and Patrick Dreher is the focus of this article, which is accompanied by online videos and slideshows.

AKOYA CULTURED PEARL FARMING IN EASTERN AUSTRALIA

Bracelet containing akoya cultured pearls from Australia
A bracelet showing naturally colored blue, cream, yellow, and silver akoya cultured pearls from Broken Bay Pearls. Photo courtesy of Broken Bay Pearls Pty Ltd.

For two decades, akoya cultured pearls in a variety of colors have been produced from P. imbricata fucata mollusks on Australia’s eastern coast. In this article, Laura M. Otter and her coauthors detail the production and conduct the first gemological and mineralogical study of these pearls. Their study concludes that colored Australian akoyas can be separated from pearls produced by other mollusk species.

ACCURATE REPORTING OF KEY TRACE ELEMENTS IN RUBY AND SAPPHIRE USING MATRIX-MATCHED STANDARDS

Figure 1. One of GIA’s new LA-ICP-MS corundum calibration standards sets (set number 1); the epoxy disk containing the set is 1.5 inches in diameter. NIST SRM 610 and 612 glass standards are included to account for trace and non-trace elements, such as those found in inclusions, that were not incorporated into the synthetic sapphire crystals grown for standards. Photo by Jennifer Stone-Sundberg.
One of GIA’s new LA-ICP-MS corundum calibration standards sets (set number 1); the epoxy disk containing the set is 1.5 inches in diameter. NIST SRM 610 and 612 glass standards are included to account for trace and non-trace elements, such as those found in inclusions, that were not incorporated into the synthetic sapphire crystals grown for standards. Photo by Jennifer Stone-Sundberg.

Trace element analysis is crucial to determining geographic origin of corundum. Jennifer Stone-Sundberg and a team of researchers have developed highly accurate calibration standards for chemical analysis at GIA using the LA-ICP-MS technique. These sets are designed for use in basic research and geographic origin reporting of ruby and sapphire.

REAL-TIME MICRORADIOGRAPHY OF PEARLS: A COMPARISON BETWEEN DETECTORS

Diagram of an II and an FPD
Structures and principles of operation for an II (left) and an FPD (right). Modified after Ide et al.

Over the past twenty years, real-time X-ray microradiography (RTX) has largely replaced film-based X-ray imaging in the examination and identification of pearls. Stefanos Karampelas and his colleagues discuss the benefits and drawbacks of two different types of detectors used with RTX units.

CHART OF INCLUSIONS IN NATURAL, SYNTHETIC, AND TREATED RUBY

Micro-Features of Ruby
These rubies, a 16.68 mm rough and a 2.72 ct cushion cut, are from Myanmar. Photo by Robert Weldon/GIA, courtesy of the William F. Larson collection.

The third wall chart in a series documents inclusions often found in natural, synthetic, and treated emerald. Inclusion specialists Nathan Renfro, John Koivula, and Jonathan Muyal provided the 30 detailed photomicrographs in this chart.

LAB NOTES

Alexandrite cabochon in fluorescent (left) and incandescent light (right).
This alexandrite cabochon appears brownish green in fluorescent light (left) and brownish purple in incandescent light (right). The cabochon shows chatoyancy under both forms of illumination due to light reflection from the shallow plane of dense fine parallel needles. Photos by Shunsuke Nagai.

Bulletins from GIA’s labs around the world include a cat’s-eye alexandrite with a unique inclusion pattern, a synthetic moissanite imitating rough diamond, and two natural sapphires with synthetic ruby overgrowth.

MICRO-WORLD

Inclusion scene of pyrite crystals and aragonite spheres in Ethiopian opal.
This Ethiopian opal contained an interesting inclusion scene consisting of a string of octahedral pyrite crystals and aragonite spheres. Photomicrograph by Charuwan Khowpong; field of view 1.40 mm.

G&G’s section on the inner world of gemstones features entries on aragonite spheres in Ethiopian opal, twinned calcite inclusions in Mogok ruby, and shattuckite in quartz.

GEM NEWS INTERNATIONAL

Sixteen samples of facet-quality ruby
Sixteen samples of facet-quality ruby (0.8–7.1 ct) from Longido, Tanzania. Photo by Sasithorn Engniwat.

The GNI section features a preliminary look at facet-grade ruby from Longido, Tanzania; an update on the Foxfire rough diamond, mined from Canada’s Diavik mine in 2015; and the testing of hydrophane opals dyed pink and blue.

2017 DR. EDWARD J. GÜBELIN MOST VALUABLE ARTICLE AWARD

G&G’s Most Valuable Article Award invites the journal’s readers to vote on the three most important articles from the previous year. Each completed ballot is entered into a drawing to win a one-year subscription to Gems & Gemology. To qualify for the drawing, please submit your votes for the 2017 MVA Award by Monday, March 12, 2018.

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