Unusual Colored Stones
Recently, a parcel of approximately 40 rare gemstones was submitted to GIA’s Carlsbad laboratory for identification. Six of these stones represented gem types that had never been examined at this location. They were identified using standard gemological testing, Raman spectroscopy, and energy-dispersive X-ray fluorescence (EDXRF).
A 0.87 ct translucent green hexagonal step cut was identified as adamite, Zn2(AsO4)(OH). It had a specific gravity (SG) of 4.40 and a refractive index (RI) of 1.721–1.750. The stone fluoresced weak green under long-wave UV light but was inert to short-wave UV, which is atypical of adamite. Clouds and fractures were observed with 30× magnification.
A translucent brown trapezoidal step cut with an SG of 5.18 proved to be bahianite, Al5SbO14(OH)2, which is especially rare as a facet-grade gemstone. The 0.73 ct bahianite had a refractive index over the limit (OTL) of the RI liquid. It was inert under long-wave UV light but displayed very weak blue under short-wave UV light. Large fractures, along with numerous whitish fibrous, radial, and granular inclusions and some surface-breaking opaque metallic inclusions, were visible using 30× magnification. It also displayed vitreous to subadamantine luster.
A 42.08 ct transparent light brownish yellow octagonal mixed-cut specimen with an SG of 6.54 displayed an OTL refractive index and a biaxial optic figure, indicating its doubly refractive nature. It was inert to long-wave UV light but fluoresced very weak yellow under short-wave UV light. Magnification revealed strong doubling, strong fire, and numerous growth tubes and needles. Its heft was high due to its lead content. These properties, along with advanced testing, led to its identification as cerussite, PbCO3. Although cerussite is a common weathering product of lead ore minerals, its softness, brittleness, and heat sensitivity make it a very rare faceted gem, particularly in this large size.
Another stone from the collection, a 0.33 ct transparent to semitransparent purple triangle step cut with an SG of 3.49 and an RI of 1.728–1.734, was determined to be chambersite, Mn3B7O13Cl. The gem was inert to both long- and short-wave UV light and showed numerous clouds with 30× magnification. Chambersite is an extremely rare mineral, found as deep as 70 feet in salt brines (J.E. Arem, “Chambersite,” The Color Encyclopedia of Gemstones, 1987, 2nd ed., p. 65). Because it is so small and dark, this material is almost never faceted.
A 2.45 ct semitransparent orange cushion modified brilliant was identified as olmiite, CaMn[SiO3(OH)](OH), which was first discovered in 2006 in South Africa (http://www.mindat.org/min-30762.html). Its SG was 2.95. The stone was doubly refractive, displayed an RI of 1.648–1.671, and fluoresced very weak orangy red under both long- and short-wave UV light. Magnification revealed wavy graining, fibrous dislocations and inclusions, and a slightly oxidized stone surface. Most olmiite crystals are not suitable for faceting.
The final stone of the six, a transparent pear-shaped modified brilliant that changed from greenish yellow under fluorescent light to yellowish orange under incandescent light, was identified as color-change remondite, Na3(Ce,La,Ca,Na,Sr)3(CO3)5. The 1.30 ct stone had an SG of 3.40 and showed an RI of 1.628–1.631, yielding a birefringence of only 0.003. It was inert to both long- and short-wave UV light. Handheld prism spectroscopy revealed a strong rare earth element spectrum. Large growth tubes, scattered particles, and etch channels were observed with 30× magnification. The stone’s rounded facet junctions indicated a low hardness. Remondite was previously reported as a color-change burbankite-related mineral from Quebec (Winter 1992 GNI, pp. 270–271).
Such a suite of gemstones, prized among avid gem collectors, is not often submitted to the laboratory. Without the help of advanced analytical equipment such as Raman spectroscopy and EDXRF to complement standard gemological testing, it would be challenging to conclude the identity of these rare gemstones.