Pyrope Garnet and Montana Sapphires Highlight Winter 2015 G&G

The lead article in this issue analyses the colour-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 exhibit the reported colour-change characteristics. The round “Super Spiral cut” specimen, weighing 15.25 ct., comes from the same material as the 42.42 ct. rough nodule. Photo by Robert Weldon/GIA, courtesy of Meg Berry

The Winter 2015 issue of Gems & Gemology offers in-depth analysis of pink pyrope garnet and alluvial Montana sapphires, a new look at a legendary sapphire once owned by Louis XIV, a study of tarnish spots in high-purity gold, and a visit to Australia’s historic opal fields.

Vanadium- and Chromium-Bearing Pink Pyrope Garnet: Characterisation and Quantitative Colorimetric Analysis

A new type of pyrope, which shows a noticeable colour change from purplish pink under incandescent light to purple in daylight, contains vanadium and chromium and is believed to originate in Tanzania.

GIA researchers Ziyin Sun, Dr Aaron Palke and Nathan Renfro performed extensive colorimetric tests on samples to analyse the colour change and the factors that influence it.

They determined the range of stone sizes that maximise the colour change, although the difference is not significant enough, according to some definitions, to be classified as an actual “colour change”.

Alluvial Sapphires from Montana: Inclusions, Geochemistry and Indications of a Metasomatic Origin

Alluvial sapphires have been mined for more than a century in Montana, but the original source rocks for this material have never been located.

A team of researchers led by Dr J. C. “Hanco” Zwaan, head of the Netherlands Gemmological Laboratory and a researcher at the Naturalis Biodiversity Center, examined inclusions in sapphires taken from the Rock Creek area to determine their possible origin. Analysis of approximately 400 samples indicated a metasomatic origin. One specimen contained a topaz crystal, the first documented topaz inclusion in sapphire.

Montana sapphires occur at three commercial secondary deposits: Rock Creek, Dry Cottonwood Creek and the Missouri River area. These rough and faceted Rock Creek stones show the range of permanent colours after routine heat treatment without the addition of chemicals. The largest faceted gem weighs 0.43 ct., and the largest rough stone weighs 1.86 ct. Courtesy of Fine Gems International, Robert E. Kane    

The Grand Sapphire of Louis XIV and the “Ruspoli” Sapphire: Historical and Gemmological Discoveries

The 135.74 ct. Grand Sapphire, a legendary gemstone acquired by Louis XIV in 1669, is housed in the National Museum of Natural History (MNHN) in Paris. Newly discovered documents about this six-sided “lozenge”-cut sapphire indicate that the king acquired the gemstone about the same time that he obtained the Tavernier Blue diamond, and that he had both mounted into gold settings in 1672. The Grand Sapphire has often been mistakenly referred to as the “Ruspoli” sapphire, a completely different gem, and archival evidence reveals the story of this confusion.  

The authors, led by Dr François Farges, professor of mineralogy at the MNHN, also determined that the sapphire probably originated in Sri Lanka.

Characterisation of Tarnish Spots in Chinese High-Purity Gold Jewellery

Part of gold’s appeal is that it generally does not tarnish − but there have been instances where red, brown or black tarnish spots have appeared on jewellery pieces of extremely high purity (99.9% Au). A team of Chinese researchers, headed by Dr Taijin Lu of the National Gems & Jewellery Technology Administrative Center (NGTC) in Beijing, determined that the spots were caused by silver deposited on the gold surface during the manufacturing process.

Splendour in the Outback: A Visit to Australia’s Opal Fields

A GIA field report from the opal mines of Australia completes the feature articles in this issue. Australia exports more than $85 million (£51 million) worth of opals annually, with some 200,000 tourists visiting the opal areas each year.

Lab Notes

In the Lab Notes column, two of the largest CVD synthetic diamonds ever submitted to GIA’s lab demonstrate the continual improvement in quality and size of this material. In addition, there are reports on a treated pink diamond with a growth structure usually associated with HPHT synthetics and large natural fossil blister pearls from giant clams found in south-eastern Kenya.

The I-colour 3.23 ct. round on the left and H-colour 2.51 ct. round on the right are the largest CVD synthetic diamonds GIA has tested. Photo by GIA


The Micro-World section presents photomicrographs of a Mexican fire agate cabochon whose colouration and structure create an unusual visual effect, a red heart-shaped pyrope garnet inclusion in a round brilliant diamond, a violetish blue spinel inclusion in a yellow sapphire, parasite inclusions in Colombian quartz, and four-ray stars in Paraíba tourmaline.

Upon close examination with a field of view of 8.12 millimetres, the polished botryoidal knob in this Mexican fire agate resembles a reptilian eye. Photomicrograph by John Koivula/GIA

Gem News International

Gem News International reports on a large fluid inclusion in a demantoid garnet from Madagascar that provides a strong clue to the source’s mineralisation. Other entries include a possible new source of grandidierite in Madagascar; a bluish green chalcedony from Africa, marketed as “Aquaprase”; and a report on ruby mining in Zahamena National Park in Madagascar.

Russell Shor is senior industry analyst at GIA in Carlsbad.