Feature Gems & Gemology, Summer 1986, Vol. 22, No. 2

Some Unusual Sillimanite Cat's-Eyes


Brown-black sillimanite cat's-eyes from Sri Lanka present an unusually sharp band, which would make them extraordinary gems were it not for the rather unattractive body color of the stones. A study of six of these stones found that they contain 0.5 wt. % iron oxide. The principal inclusion mineral is ilmenite, which occurs in elongated, submicroscopically thin lamellae. Complex thicker lamellae consist of hercynite spinel grown together with a member of the pyroxene group. These thicker inclusions were not found in all of the specimens investigated.