Sillimanite

Sillimanite is formed from aluminum silicate and is a transparent to translucent mineral ranging from white to gray, brown and green. It is mostly found as silky and fibrous masses with glassy luster. If it is not fibrous, the best field indicators are its crystal habit, color, brittleness and hardness.

Sillimanite is named after an American chemist and mineralogist, Prof. Benjamin Silliman. For its fibrous inclusions, it is sometimes also known as "fibrolite" in the jewelry world. Gem quality sillimanite crystals are rare and is seen seldom at retail.

Blue variety of sillimanite is the most sought and is found only in the ruby-rich Mogok areas of Burma. The gray to green variety is found in Sri Lanka from the gem gravels. A third variety, usually used in the creation of artifacts, is found in the Clearwater River Valley in Idaho.

Sillimanite is basically used to manufacture mullite refactory ceramics, spark plugs, and high temperature crucibles. For jewelry, the chatoyant green and blue varieties are most suitable. The hardness of the stone is 6 - 7 on Mohs scale and specific gravity, 3.23.

Techinical Specification

Name	Moh's	Specific Gravity	Structure	Cleavage	Refractive Index	Double	Disp.	Pleochro	Fluorescence
Sillimanite	6-7.5	3.25	e	f	g	h	i	j	k