Lab Notes
Gems & Gemology, Summer 2017, Vol. 53, No. 2

Cat’s-Eye Kornerupine

Akhil Sehgal and Daniel Girma
Kornerupine with strong chatoyancy.
Figure 1. This 1.44 ct yellowish green kornerupine displayed strong chatoyancy. Photo by Sood Oil (Judy) Chia.

A dark, translucent, yellowish green oval cabochon (figure 1) was recently submitted to the New York lab for identification. The 1.44 ct stone displayed an intense chatoyancy. With a spot refractive index reading of 1.67 and a hydrostatic specific gravity of 3.32, the stone was identified as kornerupine, a rare borosilicate mineral found in boron-rich volcanic and sedimentary rocks that have undergone metamorphism (http://rruff.info). Raman spectroscopy confirmed the identification.

Cat’s-eye kornerupine is an extremely rare gemstone. Its chatoyancy is caused by rutile and graphite inclusions (H.N. Lazzarelli, Gemstones Identification: Blue Chart, 2010, www.gembluechart.com). However, we were unable to identify the needles in this stone using Raman spectroscopy. Fiber-optic illuminated magnification exposed dense clusters of these needles, some displaying iridescence (figure 2). This was GIA’s first encounter with this material in more than a decade.

Dense collection of needle inclusions causing chatoyancy.
Figure 2. A dense collection of needle inclusions was the cause of the chatoyancy.
Photo by Daniel Girma; field of view 2.08 mm.

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