how to facet
gemstone main cabochon creation
mineral data
Mineral Name: sillimanite
Gem Names: sillimanite - buchoizite - fibrolite
Chemistry: Al2SiO5
Class: silicate - neosilicate
Crystal Sys: orthorhombic
Hardness: 6.5 - 7.5 Specific Gravity: 3.24
Fracture: splintery Refractive Index: 1.65 - 1.684
Dispersion: 0.015 (strong)
Misc Prop: Sillimanite is one of the three polymorph minerals, andalusite, sillimanite and kyanite. They have the same chemical formula but different crystal structures. It is rare to find sillimanite crystals clear enough or large enough to facet.

It, like andalusite, is a pleochroic mineral showing two colors in finished stones. It has perfect cleavage in one direction which makes it a bit of challenge for faceters. The colors are typically clear, green, and brown, but occasionally a blue is found.

Because of its fibrous nature some varieties are cut into cabochons with catseyes. It is a rare mineral in jewelry and more often a collectors stone.

Color: In gemstones, color, is often the key to naming or describing the particular variety. Hence look for this information below in the various descriptions. Color, opacity, and homogeneity often determine the placement of value on any given stone, and are all optical properties of the particular stone.
Specific Data:
faceted stone