Winter 2011

Featured Gem News International: Vanadium-Bearing Color-Change Sphene from Pakistan/Afghanistan

Recently seen in the Carlsbad laboratory was a parcel of rough and cut sphene loaned for examination by Eric Braunwart (Columbia Gem House, Vancouver, Washington). According to him, the sphene originated near the Pakistan/Afghanistan border. The most interesting feature of this material was that many of the stones showed a slight to moderate color change. In daylight-equivalent lighting, the sphene was dominantly vivid green to yellowish green, changing to brownish orange or brown under incandescent light. However, strong pleochroism was responsible for the multiple colors seen in the faceted stones.

The rough consisted of well-formed blade-like crystals, so it is apparent that the material was mined from a primary deposit. Rough production as of July 2011 was estimated by Mr. Braunwart to be around 600 g, from which approximately 200 carats of finished material have been produced. The cut stones ranged from calibrated sizes as small as 3 mm in diameter up to larger stones weighing several carats. The largest faceted stone from the production so far was a 5.45 ct oval brilliant.

Gemological properties of the material were consistent with sphene. The RI was over-the-limit of the refractometer, and the average SG (measured hydrostatically) was 3.54. A strong red reaction was observed with a Chelsea filter. Microscopic observation revealed strong doubling and numerous included crystals. These were identified by Raman microspectroscopy as transparent near-colorless apatite and dark elongated needle-like crystals of an amphibole mineral. Several stones also contained fluid “fingerprints.” A few rough pieces showed prominent zoning, and the color change was less apparent across these zoned areas.

EDXRF spectroscopy of all samples showed major amounts of Ca, Ti, and Si that are expected for sphene, as well as traces of V, but no Cr. LA-ICP-MS measurements of several spots on a faceted sample detected an average of more than 2400 ppmw V (and no Cr), which is presumably the cause of the color change. The visible spectrum showed a broad absorption feature centered at approximately 603 nm. Transmission windows on either side of this broad band are consistent with the spectrum expected for a color-change gem.

The coloration of this sphene is much different from the typically “golden” orange material previously described from Pakistan’s North West Frontier Province (Spring 2006 GNI, pp. 68–69), and also from the yellow sphene known from Badakhshan, Afghanistan (Summer 2006 GNI, pp. 180–182).

Nathan Renfro