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It was named after the American chemist and mineralogist, Professor Benjamin Silliman (1779-1824). Sometimes referred to as "Fibrolite", the crystals occur in groups, resembling fibers. Sillimanite is a transparent to translucent mineral that ranges in color from white to gray to brownish to greenish.


Sillimanite is formed from aluminum silicate and is mostly found as silky, fibrous masses and as transparent prismatic crystals with glassy luster. Best field indicators of Sillimanite are its crystal habit, color, brittleness and hardness if not fibrous.

Physical Properties:


Sillimanite Birthstone

gray, green, white, brown, grayish blue
6 - 7
Specific Gravity
Refractive Index
1.64 - 1.68


Sillimanite is used in the manufacturing of mullite refactory ceramics, spark plugs, and high temperature crucibles. This stone will help one to realize their purpose and can stimulate intuition.The aluminum silicate mineral sillimanite, used in the manufacture of spark-plug porcelain.Sillimanite is suitable for lapidary work.


Sillimanite is widespread throughout the schists of the Delaware Piedmont area of the U.S and occurs as large masses and steam-rounded boulders at the Brandywine Springs State Park. Under the name Fibrolite, it was recognized by geologists in Delaware prior to 1830.


Brazil, central Europe, Myanmar, Sri Lanka, Czech Republic, India, Italy, Germany and Idaho in the United States.