SILLIMANITE

SILLIMANITE	Polymorph
Major Sources	Brazil, India & Sri Lanka
Colors Found	Blue, brown, colorless, green, white & yellow
Family	Sillimanite: Al2SiO5
Hardness	6 to 7
Refractive Index	1.65–1.68; Biaxial (+)
Specific Gravity	3.24
Crystal System	Orthorhombic
Enhancements	Natural

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 mineral. 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 (the ‘cat’s eye effect,’ caused by parallel inclusions reflecting a single band of light back to the eye) and asterism (the ‘star effect,’ caused by intersecting inclusions reflecting a star of light back to the eye). These optical effects are highly coveted and well-suited to cabochon rings and earrings.