Microworld Gems & Gemology, Summer 2015, Vol. 51, No. 2

Iridescent Musgravite

Nathan Renfro

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Gray musgravite displaying iridescence.
Figure 1. This 0.86 ct gray musgravite displays an unusual iridescent phenomenon that is clearly visible in the table facet. Photo by Kevin Schumacher.

Musgravite is a rare mineral closely related by composition to the mineral taaffeite. This magnesium-rich beryllium oxide crystallizes in the trigonal system, in contrast to the hexagonal system of taaffeite, and is highly sought after by rare stone collectors. A 0.86 ct musgravite, identified by Raman spectroscopy, contained a particularly interesting inclusion scene consisting of numerous etch tubes that broke the surface of the faceted stone (figure 1). With a direct source of light, these etch tubes displayed vibrant colors resulting from thin-film iridescence in the air-filled, crystallographically aligned tubes (figure 2). This is the first musgravite gemstone displaying any type of optical phenomenon that this author has examined to date.

Thin-film iridescence of musgravite viewed using oblique fiber-optic illumination.
Figure 2. Thin-film iridescence along crystallographically oriented etch tubes was the source of the colorful phenomenon in this rare musgravite gemstone, viewed using oblique fiber-optic illumination. Field of view 2.47 mm. Photomicrograph by Nathan Renfro.

Nathan Renfro is the analytical manager of the gem identification department, and analytical microscopist in the inclusion research department, at GIA in Carlsbad, California.