Alongside the more common andalusite and kyanite, sillimanite is one of three aluminosilicates that may occasionally be fashioned into gemstones. The mineral also has many important industrial uses relating to ceramics, glass-making, and the smelting of iron and steel.

While transparent, light-toned grayish, yellowish, brownish, greenish and/or bluish crystals are sometimes faceted, the most commonly encountered gem sillimanites are the cabochon-cut cat’s eye specimens. These are usually formed from a fibrous sub-variety known as “fibrolite”, and often possess a translucent dark gray or opaque reddish-brown coloration. However, near-colorless chatoyant material is also encountered with significantly higher levels of transparency.

Named after the nineteenth century American chemist Benjamin Silliman, most gem-quality sillimanite is sourced in Sri Lanka, India, Kenya and Myanmar’s famous Mogok gemstone tract. In addition, it has also been designated as the official state mineral of Delaware.