Tom Schneider, a colored stone dealer in San Diego, California, recently shared the unusual phenomenal gem shown in figure 12 with the West Coast lab. The 8.19 ct red cabochon showed a true double star: two six-rayed stars slightly offset from each other (rather than a 12-rayed star).

The refractive index of 1.76 and the absorption spectrum proved that the gem was ruby. (Recall that the ruby spectrum shows lines in the red at about 659 and 668 nm; a strong doublet at 692 and 694 nm; and lines at 468 and 475 nm.) The pervasive inclusions of rutile needles (typically referred to as 'silk') indicated a natural stone. However, the double star seemed illogical: The normal configuration of silk in a ruby should produce a single star from one viewing direction with one light source.

Further observation with magnification revealed the cause of the double star. The ruby was twinned throughout, with closely spaced lamellar twinning planes. The silk was slightly offset across each of these planes, as shown in figure 13, because of the difference in the orientation of the twins; thus, the twinning produced silk that was aligned in every other plane. This offset led to the appearance of two stars slightly separated from each other. In addition, when the rays of the stars were viewed with magnification, they had small gaps (like a dotted line); they looked continuous to the naked eye because the spacing between these many twin planes was so small.

Photo Shane McClure