Fall 2008

Featured Gem News International: Colorless Forsterite from Tajikistan

In addition to the gem-quality colorless forsterite from Myanmar now circulating in the trade (see Fall 2008 Gem News International, pp. 263–265), colorless forsterite from Tajikistan recently became available. According to gem dealer Farooq Hashmi (Intimate Gems, Jamaica, New York), small quantities of this material have been produced from the Kukh-i-Lal skarn-type gem deposit in the southwestern Pamir Mountains, where it is found in association with clinohumite and spinel. Mr. Hashmi noticed a few of the colorless pieces of rough mixed with a 3+ kg parcel of clinohumite that he obtained in mid-2008. He cut a 3.54 ct gem from one of these pieces, and loaned it to GIA for examination.

Gemological characterization of this colorless pear-shaped brilliant (figure 1) yielded RIs of 1.637–1.669 (birefringence 0.032), a biaxial optic figure, and a hydrostatic SG of 3.26. It fluoresced weak orangy pink to short-wave UV radiation and weak red to long-wave UV. In addition to major amounts of Mg and Si, EDXRF spectroscopy revealed traces of Ca, Mn, and Fe. These properties are generally consistent with those reported for natural colorless forsterite (Spring 1999 Lab Notes, pp. 49–51; and K. Nassau, "Synthetic forsterite and synthetic peridot," Summer 1994 Gems & Gemology, pp. 102–108). The elements detected by EDXRF were confirmed with laser ablation–inductively coupled plasma–mass spectroscopy (LA-ICP-MS; calibrated with NIST glass standards), which showed average concentrations of 43.7 wt.% MgO and 0.34 wt.% FeO. As Mg is strongly dominant over Fe, the stone’s composition lies near the forsterite end of the forsterite-fayalite series.

Microscopic examination showed a "fingerprint" composed of euhedral-to-subhedral inclusions (figure 2). With higher magnification and diffused lighting, these inclusions were observed to contain a white, fine-grained solid and—in some cases—a dark opaque crystal with a hexagonal outline.

UV-Vis spectroscopy revealed weak absorption features at 451, 473, and 491 nm. Raman spectra taken with 488 and 514 nm laser excitation matched the forsterite and peridot spectra from our database. A 514 nm Raman scan was also taken up to 6000 cm-1 to investigate the fluorescence behavior. This showed a broad band with the maximum at approximately 4000 cm-1, which is essentially equivalent to the fluorescent band reported for Burmese forsterite.

David M. Kondo
GIA Laboratory, New York