Beautiful and rare, Sillimanite is named for the famous American geologist Benjamin Silliman (1779-1864). Sillimanite is sometime referred to as Fibrolite. Sillimanite is not only scarce, but also difficult for miners to identify and problematic for cutters. These three attributes combine to ensure that Sillimanite remains a true exotic gemstone.

Just the facts

Sillimanite is formed from aluminum silicate and is usually found as silky, fibrous crystals suitable for cabochons. Transparent crystals suitable for faceting display a glassy luster and are exceedingly scarce. Sillimanite is colorless, white, brown, yellow, blue and green in color.

Sillimanite is typically found scattered within layers of metamorphic rocks that have been put under great pressure and high temperature. This is why Sillimanite is commonly found in volcanic or hot spring areas. Because of the way it is scattered within the host rocks, miners often have difficulty in detecting Sillimanite.

Sillimanite is a polymorph with two other minerals, Kyanite and Andalusite. A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals. This is unusual, and due to its brittleness, Sillimanite is very difficult to facet. Some 50% of gem quality crystals can be damaged during the faceting and fashioning process alone!

Some Sillimanite crystals demonstrate chatoyancy (also known as the “cat’s eye effect,” caused by minerals reflecting a single band of light back to the eye) and asterism (also known as the “star effect,” this is caused by minerals reflecting a star of light back to the eye). These optical effects are highly coveted and well-suited to cabochon rings and earrings.

SILLIMANITE Polymorph
Origins Brazil, India & Sri Lanka
Colors Found Blue, brown, colorless, green, white & yellow
Family Sillimanite
Hardness 6.00 – 7.00
Refractive Index 1.65 – 1.68
Relative Density 3.24
Source: GemsTV.com

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