GEM NOTES

John J. Bradshaw, GG (GIA)

GEMGUIDE - 1 - JULY / AUGUST 2017

INTRODUCTION

At Lotus Gemology, our Bangkok-based gem testinglaboratory, we only test for two types of stones:corundum (ruby & sapphire) and spinel. As a result,

we often get parcels of stones with similar attributes. Thismeans that sometimes we can test twenty or thirty un-treated Mozambique rubies in a row, for example. Often itis in the routine testing of these seemingly uniform parcelsthat we uncover surprises.

We recently got one such lot of 11 light yellow sapphires,ranging from approximately 2–5 ct. This is a standard, un-remarkable submission for our lab. Figure 1.

TESTINGAppearanceThe appearance of the 11 stonesin the lotwas relatively homoge-nous. All displayed a pleasant yel-low hue, with rich to pastelsaturation, and all had a medium-light to light tone. Even the cuttingwas similar, with most stones inthe parcel cut with a modified bril-liant crown and either fancy orstep cut pavilion. At Lotus, yellowsapphires with this appearanceare common.

UV FluorescenceAfter noting the usual identifyingfeatures of the stones, includingweight, measurements, and ap-pearance, we checked the stones’ reactions in fluorescentlighting.

Ten of the yellow stones in the lot displayed similar fluores-cence patterns with medium yellow to orange fluorescenceacross the stone in the longwave light, and weak to mediumorange fluorescence in the shortwave light.

In contrast, one stone had a medium red-orange fluores-cence in the long wave and weak reddish-orange fluores-

cence in the shortwave. Under both wavelengths, the fluo-rescence of our subject was markedly redder than that of theother yellow sapphires in the lot. This was the first sign ofsomething unusual.

Visible SpectrumUsually in natural yellow sapphire, as was the case withmost of the stones submitted in this parcel, a weak Fe(iron) spectrum can be seen. However, the stone with thereddish fluorescence did not display a diagnostic spectrum.

This could be cause for concern, yet detecting a weak ironspectrum using the direct-vision spectroscope can be

tricky for many gemologists. (Tolearn more, see our article “GemTesting with the Spectroscope.”)In fact, a weak line at about 455nm has even been reported insynthetic flame fusion corun-dum (Koivula, 2005). Thus, moretesting had to be done.

Infrared SpectrumWhen tested with the FTIR(Fourier transform infrared spec-troscopy), 9 of the 11 stones sub-mitted displayed a weak tostrong peak at 3160 nm. Twospectra did not display diagnosticfeatures, including the one asso-ciated with our subject.

It is a little suspicious that thesetwo spectra stood out from the rest, but that is not diag-nostic evidence when viewed alone. Because of factorssuch as differences in cutting, some stones may not dis-play a strong signal that provides a clear, diagnostic spec-trum. Figures 2 and 3.

InclusionsMost of the stones in the lot displayed inclusions typical ofuntreated Sri Lankan sapphire. These include angular zonedclouds of exsolved particles, angular growth zoning, partially

A SAPPHIRE’S SECRET

FIGURE 1. The 4+ ct yellow stone that is the subject of this report. Photo: Maitree Petchloun.

GEM NOTES

GEMGUIDE - 2 - JULY / AUGUST 2017

healed fissures (fingerprints), euhedral negative crystals, andsmall uraninite crystals.

The stone in question was almost entirely free of inclusions.The only inclusions we could find using dark field lighting inthe microscope were a few shallow fissures and a series ofsmall pinpoint dots near the culet. Because of their small size,it was hard to tell whether these dots were exsolved parti-cles, tiny crystals, or gas bubbles. Figure 4.

To do further testing, we immersed the stone iniodomethane (methylene iodide) under crossed polars. Wethen saw iridescent rainbow lines dancing across the stone.With a cursory glance this could be mistaken for angulargrowth zoning. However, after rotating the stone in different

directions, we could see that these features actually followedparallel twinning that intersected at 60° or 120° angles,termed Sandmeier-Plato twinning. These twin planes formparallel to the hexagonal prism and are a diagnostic featureof flame fusion synthetic corundum (Eppler, 1964; Gübelin,1986, 2005; Hughes, 2017). Figure 5.

ChemistryWe also analyzed the chemical composition of the stone inquestion using EDXRF (energy dispersive x-ray fluorescence).

Two features stood out that confirmed our suspicions.One was the complete lack of Ga (gallium) at 0.000%. Theother was the extremely low level of Fe (iron) at 0.0010%. In natural sapphire we would expect to see higher lev-

FIGURE 3.This is the spectrum of the subject of this report. It did not showany diagnostic features at 3160nm.

FIGURE 2.The above spectrum shows a strong 3160nm peak typical of thenatural sapphires in the parcel.

FIGURE 4. Small, bright scattered dots are visible under the culet of thisstone. Upon further testing, we determined that they are gas bubbles, a com-mon feature of Verneuil synthetic corundum. (The abrasions at the intersec-tion of the facet edges are small chips at the culet.) Photo: E. Billie Hughes.

FIGURE 5. Sandmeier-Plato twinning is a type of polysynthetic twinning parallel to the hexagonal prism. These lines are proof of Verneuil syntheticorigin. Photo taken under crossed polars while the stone was immersed in iodomethane (methylene iodide). Photo: E. Billie Hughes.

Element Weight %

Al2O3 99.9956

Cr2O3 0.0033

Fe2O3 0.0010

Ga2O3 0.0000

Gemworld International, Inc., 2640 Patriot Blvd, Suite 240, Glenview, IL 60026-8075, www.gemguide.com© 2017 Gemworld International, Inc. All rights reserved.

All articles and photographs that appear are copyrighted by the author, the contributing person or company, or Gemworld International, Inc. and may not be reproduced in any printed or electronic format, posted on the internet, or distributed in any way without written permission. Address requests to the editor-in-chief.

The opinions expressed in this publication are the opinions of the individual authors only and should not necessarily be considered to be the opinions of the staff of Gemworld International, Inc.as a whole. Any website listings that appear in articles are for informational purposes only and should not be considered an endorsement of that company.

GEM NOTES

GEMGUIDE - 3 - JULY / AUGUST 2017

els of both of these elements (Muhlmeister, 1998;Hughes, 2016). Figure 6.

CONCLUSIONIt is not any single gemological test that tells us the true na-ture of a stone. Rather, it is the combination of evidence froma series of test, which, when seen as a whole, helps us forma conclusion.

In this instance, the unusual red fluorescence, coupled witha lack of diagnostic visual and infrared spectra, raised our sus-picions about the nature of the stone. These suspicions werethen confirmed when we found gas bubbles and Sandmeier-

Plato twinning with microscopic observation. Chemical analy-sis further solidified the evidence that this seemingly typicalyellow sapphire was actually a flame fusion synthetic, hiddenamong natural stones. u

ACKNOWLEDGEMENTSThe author would like to thank Richard W. Hughes, EliseSkalwold, and Patharaphum Sudprasert for their help.

REFERENCES & FURTHER READINGEppler, W.F. (1964) Polysynthetic twinning in syntheticcorundum. Gems & Gemology, Vol. 11, No. 6, Summer, pp.169–175.

Gübelin, E.J. and Koivula, J.I. (1986) Photoatlas of Inclu-sions in Gemstones. Zürich, Switzerland, ABC Edition, revised Jan., 1992

Gübelin, E.J. and Koivula, J.I. (2005) Photoatlas of Inclu-sions in Gemstones, Volume 3. Basel, Switzerland, OpinioPublishers, 672 pp.

Hughes, E. B., Chankhantha, C. et al. (2016) Padparadschaor Pretender: An Unusual Pink-Orange Sapphire. The Aus-tralian Gemmologist, Vol. 25, Nos.11–12, pp. 389-392.

Hughes, R.W., Manorotkul, W. & Hughes, E. B. (2017) Ruby & Sapphire: A Gemologist’s Guide. Lotus Publishing,Bangkok, 816 pp.

Koivula, J.I. &Hughes, R.W. (2015) Gem testing with thespectroscope. Lab World Magazine, Vol. 04, No. 04, May–July, pp. 05–08

Muhlmeister, S., Fritsch, E. et al. (1998) Separating natu-ral and synthetic rubies on the basis of trace-elementchemistry. Gems & Gemology, Vol. 34, No. 2, Summer,pp. 80-101

FIGURE 6.The chemical analysis for subject of this report revealed very lowlevels of iron and no traces of gallium. This suggests that the specimen issynthetic.