Another Look at Detection of CVD Synthetic Diamonds – and Some Unusual Red Gemstones

By Russell Shor

In the Spring 2004 issue of Gems & Gemology, researchers from the De Beers DTC (Diamond Trading Company) Research Centre discuss how synthetic diamonds grown by chemical vapor deposition (CVD) can be distinguished from natural diamonds by various gemological and laboratory tests.

DTC researchers examined more than 1,000 single-crystal CVD synthetic diamonds grown by Element Six (formerly De Beers Industrial Diamond Division) since the early 1990s. The samples tested fell into four groups: nitrogen doped (faint to dark brown), HPHT-treated nitrogen doped (near-colorless and green), boron doped (light to dark blue) and high purity (that is, without any nitrogen, boron, or other impurities – colorless to near colorless). The researchers had several samples faceted for experimental purposes only. They employed a number of techniques including microscopy; X-ray topography; UV, visible and infrared spectroscopy; photo-luminescence; cathodoluminescence; and Raman analysis; as well as testing on three De Beers Verification Instruments (the DiamondSure, DiamondView and DiamondPLus).

The CVD synthetic diamond samples showed a range of different features typically not seen in natural diamond, including distinctive absorption features, growth patterns and color zoning. The authors feel, however, that the most reliable technique is luminescence imaging, such as that achieved with the DiamondView instrument, and X-ray topography, which may give supportive evidence. The DiamondSure and DiamondPLus instruments also provide useful identification information. The Diamond Trading Company is  in negotiations for the commercial distribution of these instruments, which heretofore have been available only in gemological laboratories.  

Winter Issue: Pink-to-Red Gemstones
The Winter 2003 issue of G&G looks at two pink-to-red gemstones: pezzottaite, a newly discovered member of the beryl group that was found in Madagascar, and red beryl from Utah.

Pezzottaite made its commercial debut at the 2003 Tucson gem shows, where it was marketed as “raspberyl” or “hot pink beryl.” Subsequent research found that, while similar to beryl, the gem material is actually a new mineral. It was named pezzottaite in honor of Dr. Federico Pezzotta of the Natural History Museum in Milan, Italy, who has done significant research on the mineralogy of Madagascar.

The article, by G&G Editor Brendan Laurs and co-authors, describes how the gem mineral was recovered from the Ambatovita region of central Madagascar in November 2002, when local miners found a large cavity that contained gem-quality pezzottaite, tourmaline and spodumene crystals. When it was realized that the refractive index was too high for beryl, samples of the material were sent to Dr. Pezzotta for examination.

The authors examined 19 samples, all of which contained fine growth tubes that can create an attractive cat’s-eye effect in cabochon material for this article. Another characteristic feature was a distinctive band in the Raman spectrum that is not known in other pink or red beryls or pale aquamarine. Other distinguishing factors are stronger pleochroism than pink tourmaline and much higher cesium and lithium contents than red beryl.

One pezzottaite sample suffered near total color loss when heated to 450°C, though heating at lower temperatures had no effect. Irradiation with gamma rays restored nearly all of its purple-pink color.

Although the debut of this new gemstone created much excitement among gem dealers, the only known commercial source of the material is a single pegmatite in Madagascar (which has been mostly mined out by local diggers using hand tools). Further production will depend on mechanized mining or the discovery of another source.

Red beryl from the Wah Wah Mountains in Utah has been mined with varying degrees of success since 1958. The article by Dr. James Shigley, Timothy J. Thompson, and Dr. Jeffrey D. Keith relates how uranium prospector Lamar Hodges discovered the deposit and worked it as a hobby until 1976, when Rex Harris and partners purchased the mining rights and worked the claims on a more regular basis for another 18 years.

Now called the Ruby Violet mine, the deposit is located about 25 miles west-southwest of Milford. The gemstones are generally red, typically with partially healed fractures and solid, fluid, or fluid and gas inclusions. In some crystals, the inclusions are so numerous that portions are translucent or opaque. The color comes from manganese and is not altered by heating or irradiation.

Although the gem is very attractive, efforts to mine and market red beryl on a larger commercial basis have met with difficulty. After the claims were leased first by Kennecott Exploration Co. (1994–1996) and then by Gemstone Mining Inc. (1997–2001), the Ruby Violet mine defaulted back to the Harris group.

Lab Notes
Recently, two round brilliant diamonds – 1.09- and 3.03-carats – were submitted to the East Coast GIA Gem Laboratory by separate clients. The graders noticed evidence of a coating (in the form of pale blue spots) when the diamonds were examined with magnification. Besides the blue spots, coated diamonds often have a grayish overall body color when viewed face-down in a standard grading environment.

The recent proliferation of corundum treatments has prompted concerns that a new process has been introduced. The GIA Gem Laboratory has examined dozens of blue sapphires from Sri Lanka that show unusual color zoning, consisting of a dark blue central zone with curved irregular edges, surrounded by a light blue to near-colorless zone. It does not conform to the outside surface of the faceted stone, and in some cases it is influenced by the natural color zoning present in the stones. The nature of this treatment is still not known, though no traces of beryllium, the element used in another controversial treatment, were found.

Gem News International
Highlights of the Gem News International section include a comparison of three important blue diamonds – the 27.64-carat Heart of Eternity, the 30.62-carat Blue Heart, and the 45.52-carat Hope – and reports on gem-quality afghanite, blue beryl from Canada, ruby and sapphires from Colombia, Peruvian blue opal, and an update on tanzanite mining at Merelani. Among the synthetics and simulants described are chatoyant glass cabochons from China, new synthetic opals from Kyocera Corp. and plastic imitations of a walrus tusk and a sperm whale tooth.