Winter-2015-Gems-Gemology

VOLUME LI
Pyrope Garnet with Color-Change Phenomenon
The Origin of Montana’s Alluvial Sapphires
The Grand Sapphire: Provenance and Properties
In Search of Australian Opal
WINTER 2015
THE QUARTERLY JOURNAL OF THE GEMOLOGICAL INSTITUTE OF AMERICA

EDITORIAL
347 Garnets, Sapphires, Opals, and Tarnish on High-Purity Gold
Duncan Pay
FEATURE ARTICLES
348 Vanadium- and Chromium-Bearing Pink Pyrope Garnet: Characterization and
Quantitative Colorimetric Analysis
Ziyin Sun, Aaron C. Palke, and Nathan Renfro
This type of pink pyrope garnet, believed to have been mined in Tanzania, shows a distinct
color-change phenomenon resulting from its unique chemical composition.
370 Alluvial Sapphires from Montana: Inclusions, Geochemistry, and
Indications of a Metasomatic Origin
J.C. (Hanco) Zwaan, Eric Buter, Regina Mertz-Kraus, and Robert E. Kane
Microscopic and chemical analysis may provide clues to the original source of Montana’s
alluvial sapphires and help distinguish them from sapphires with overlapping properties.
392 The Grand Sapphire of Louis XIV and the “Ruspoli” Sapphire:
Historical and Gemological Discoveries
François Farges, Gérard Panczer, Nassima Benbalagh, and Geoffray Riondet
Archival research and gemological characterization reveal the true story of this 135.74 ct blue
sapphire from the French crown jewels.
NOTES AND NEW TECHNIQUES
410 Characterization of Tarnish Spots in Chinese High-Purity Gold Jewelry
Taijin Lu, Jian Zhang, Yan Lan, Ying Ma, Hua Chen, Jie Ke, Zhenglong Wu, and Miaomiao Tang
Examines the origin and nature of these tarnish spots and how they can be avoided.
FIELD REPORTS
418 Splendor in the Outback: A Visit to Australia’s Opal Fields
Tao Hsu, Andrew Lucas, and Vincent Pardieu
Documents a trip to the renowned opal fields of Lightning Ridge, Koroit, Yowah, and Quilpie.
REGULAR FEATURES
428 Lab Notes
Graphite inclusions in diamond • Treated pink diamond with HPHT synthetic growth
structure • Very large type Ib diamond • Uranium in hyalite • Large natural fossil blister
pearls from Tridacna species • Pearl with an intriguing internal structure • Pearls reportedly
from Spondylus and Trochoidea species • Two large CVD-grown synthetic diamonds
• Synthetic rock crystal quartz bangle with unusual inclusions
441 G&G Micro-World
“Dragon’s eye” fire agate • Red heart inclusion in diamond • Trapiche muscovite
• Parisite in Colombian quartz • Violetish blue spinel in yellow sapphire • Stars in Paraíba
tourmaline
446 Gem News International
Demantoid garnet with giant fluid inclusion • Grandidierite from Madagascar • Mexican
demantoid from new deposits • New natural-color bluish green chalcedony • Rubies
from new deposit in Madagascar • Plastic imitation of golden coral • Conference reports
• Errata
463 Letters
Winter 2015
VOLUME 51, No. 4
pg. 371
pg. 352
pg. 443
pg. 392

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About the Cover
The lead article in this issue analyzes the color-change phenomenon displayed by a pink pyrope garnet believed
to be of Tanzanian origin. The cover photo shows two pieces of pyrope garnet from Morogoro, Tanzania, that ex-
hibit the reported color-change characteristics. The round “Super Spiral cut” specimen, weighing 15.25 ct, comes
from the same material as the 42.42 ct rough nodule. The gems were photographed on a glass tile that provides
an organic look to the scene. Photo by Robert Weldon/GIA, courtesy of Meg Berry.
Printing is by L+L Printers, Carlsbad, CA.
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EDITORIAL GEMS & GEMOLOGY WINTER 2015 347
Duncan Pay | Editor-in-Chief | dpa[email protected]
Welcome to the final Gems & Gemology of 2015. Colored gems—garnet, sapphire, and opal—
are prominent in this issue. The Tanzanian garnet on our cover projects a rich, bold pink and
demonstrates just how fine this material can be. It’s profiled in our lead article. The issue also
offers insight into the origin of Montana’s alluvial sapphires, a gemological study of the historic
Grand Sapphire, an investigation into tarnish spots on high-purity gold, and a field report on
Australian opal.
Our first paper—by three of GIA’s young research scientists, Ziyin “Nick” Sun, Aaron Palke, and
Nathan Renfro—examines interesting
“color-change” pyrope garnet from
Tanzania. Under fluorescent-equivalent
lighting, larger gems have the color of
fine purple amethyst, while under incandescent they resemble hot pink tourmaline. The authors provide a comprehensive
review of this material’s inclusions along with a quantitative analysis of its color under different lighting conditions.
Alluvial sapphires from the U.S. state of Montana have long been celebrated for their kaleidoscope of colors, but so far no one
has located these gems’ primary source rocks. In our second paper, a team of researchers led by Dr. J.C. “Hanco” Zwaan
focuses on mineral inclusions and geochemistry to propose a metasomatic origin for Montana’s alluvial sapphires.
Next, Dr. François Farges and his colleagues profile the fascinating history and gemology of Louis XIV’s 135.74 ct Grand
Sapphire, which was added to the French crown jewels in 1669, around the same time as the fabled Tavernier Blue diamond.
On the basis of microscopic examination and spectroscopy, the authors suggest that this magnificent blue sapphire
originated in Sri Lanka’s gem fields.
Our fourth paper investigates unsightly tarnish spots in high-purity (99.9% Au) gold jewelry. Dr. Taijin Lu and his team
from the National Gems & Jewellery Technology Administrative Center (NGTC) in Beijing use a combination of
advanced techniques to detect the presence of silver and sulfur in the tarnish spots. The authors recommend careful
cleaning during the gold manufacturing process to eliminate the possibility of silver
contamination.
In June 2015, a GIA team consisting of Tao Hsu, Andrew Lucas, and Vincent Pardieu visited
four important opal fields at the rugged fringes of Australia’s arid Great Artesian Basin:
Lightning Ridge, Koroit, Yowah, and Quilpie. Their field report documents mining activity
and the growing importance of the revenue and employment brought to these remote outback
communities by “opal tourism.”
As usual, you’ll find our regular Lab Notes and Gem News International sections along with the
third installment of the Micro-World column. Our GNI section includes an update from field
gemologist Vincent Pardieu on rubies from a new deposit in Zahamena National Park, Madagascar.
There are also reports from the 2015 Geological Society of America (GSA) annual meeting in
Baltimore, the first International Emerald Symposium in Bogotá, Colombia, and the 34th
International Gemological Conference (IGC) in Vilnius, Lithuania.
Finally, you’ll notice this issue includes a ballot card for the Dr. Edward J. Gübelin Most
Valuable Article Award. We had a fantastic response to our 2014 reader ballot, so please do vote
again for your favorite 2015 articles.
Please enjoy the Winter issue!
Garnets, Sapphires, Opals, and
Tarnish on High-Purity Gold
Amethyst purple under fluorescent-equivalent
lighting, hot pink under incandescent...

348 “COLOR-CHANGE” PINK PYROPE GARNET GEMS & GEMOLOGY WINTER 2015
Garnets are a group of isometric nesosilicates
with the general chemical formula X3Y2Z3O12.
X, Y, and Z represent dodecahedral, octahedral,
and tetrahedral sites in the crystal structure, respec-
tively. Natural rock-forming silicate garnets with the
Z-site occupied by Si4+ are commonly divided into the
pyralspite and ugrandite groups. In pyralspite, Al3+ oc-
cupies the Y-site and the X-site may contain Mg2+, Fe2+,
or Mn2+; these garnets are dominantly composed of
the pyrope (Mg3Al2Si3O12), almandine (Fe2+3Al2Si3O12),
and spessartine (Mn3Al2Si3O12) end members. The
ugrandite garnets have Ca2+ on the X-site and Cr3+, Al3+,
or Fe3+ on the Y-site, giving uvarovite (Ca3Cr2Si3O12),
grossular (Ca3Al2Si3O12), or andradite (Ca3Fe2
3+ Si3O12)
end members. Stockton and Manson (1985) proposed
a classification scheme for separating the pyralspite
group into the gemological species of pyrope, alman-
dine, spessartine, pyrope-almandine, pyrope-spessar-
tine, and almandine-spessartine. Previously, two types
of color-change garnets have been reported: pyrope
with very high Cr3+ (Hysingjord, 1971; Carstens, 1973;
Amthauer, 1975) and pyrope-spessartine containing
both Cr3+ and V3+ (Crowningshield, 1970; Jobbins et al.,
1975; Schmetzer and Ottemann, 1979; Manson and
Stockton, 1984; Johnson and Koivula 1998; Krzem-
nicki et al., 2001; Schmetzer et al., 2009). Cr3+-bearing
pyrope, however, has not been observed in sizes large
enough for faceting. In the trade, almost all color-
change garnets are referred to as pyrope-spessartine.
A noticeable and attractive color change under
VANADIUM- AND CHROMIUM-BEARING
PINK PYROPE GARNET:
CHARACTERIZATION AND QUANTITATIVE
COLORIMETRIC ANALYSIS
Ziyin Sun, Aaron C. Palke, and Nathan Renfro
FEATURE ARTICLES
A type of pink pyrope garnet containing vanadium and chromium, believed to have been mined in Tan-
zania, appeared at the 2015 Tucson shows. The material shows a noticeable color difference from pur-
plish pink under incandescent light (A) to purple under daylight-equivalent light (D65). This study reports
a quantitative analysis of the difference in color between the two lighting conditions, based on the use
of high-quality visible absorption spectroscopy to calculate CIELAB 1976 colorimetric coordinates. L*,
a*, and b* colorimetric parameters were calculated for a wide range of path lengths as extrapolated
from visible absorption spectra of thinner samples. Using this method, the path length of light through
the stone that produces the optimal color difference can be calculated. This path length can then be
used to determine the optimal depth range to maximize color change for a round brilliant of a specific
material. The pink pyrope studied here can be designated as “color-change” garnet according to certain
classification schemes proposed by other researchers. In many of these schemes, however, the material
fails to exceed the minimum requirements for quantitative color difference and hue angle difference to
be described as “color-change.” Nonetheless, there is no simple solution to the problem of applying
color coordinates to classify color-change phenomena. Also presented is a method by which spectra
can be corrected for reflection loss and accurately extrapolated to stones with various path lengths.
See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY, Vol. 51, No. 4, pp. 348–369,
http://dx.doi.org/10.5741/GEMS.51.4.348.
© 2015 Gemological Institute of America