Characteristics of Sillimanite:

Chemical Name: Aluminosilicate

Formula: Al₂OSiO₅

Colors: Colorless, blue, yellow, green, violet

Structure: Orthorhombic

Hardness: 7

Specific Gravity: 3.2-3.3

Refractive Index: 1.66-1.68

Lustre: Vitreous to silky

Streak: White

Locations: Myanmar, Indi, Czech Republic, Sri Lanka, Italy, Germany, Brazil, USA

What is Sillimanite? 

Although mainly an industrial material, transparent Sillimanite is the basis of attractive faceted gemstones. Cabochons are cut from a form of Sillimanite called fibrolite, so named because the mineral resembles bunches of fibers twisted together. Crystals are long, slender, and glassy, or in blocky prisms. Blue and violet are the most prized colors for gemstones. Sillimanite is distinctly pleochroic: yellowish-green, dark green, and blue can be seen within the same stone from different angles. It is a common mineral in some metamorphic rocks.