Micro-World
Gems & Gemology, Summer 2017, Vol. 53, No. 2

Cosalite in Quartz

John I. Koivula
Quartz with abundance of inclusions.
Figure 1. This 16.20 ct quartz contains an abundance of inclusions. Photo by Kevin Schumacher.

A search of the gemological literature shows that rock crystal quartz contains a wide variety of unusual and interesting inclusions. Many of those inclusions look alike, so careful analysis is necessary for a positive identification. We recently examined a 16.20 ct cushion-shaped rectangular modified step-cut rock crystal from Kara-Oba, Betpaqdala Plateau, Kazakhstan (figure 1), that was fashioned by Michael E. Gray (Coast-to-Coast Rare Stones, Mendocino, California). Measuring 18.15 × 13.24 × 10.60 mm, it hosted several opaque silvery gray stalks and needles that appeared to have an orthorhombic morphology (figure 2). A fragment cut from the original rough crystal was analyzed by laser Raman microspectrometry and yielded a possible identification as cosalite, a rare orthorhombic lead bismuth sulfide. This was subsequently confirmed by X-ray powder diffraction. The discovery of such a rare mineral as an inclusion in this stone was a pleasant surprise.

The inclusions were identified as cosalite.
Figure 2. Displaying an orthorhombic morphology, these inclusions were identified as cosalite. Photomicrograph by Nathan Renfro; field of view 4.11 mm.

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