Sillimanite: Gemstones Information

Sillimanite (name preferred to fibrolite) with andalusite and kyanite is a trimorph of Al2SiO5. Transparent, rather slate-blue or blue-green slender poorly terminated prismatic crystals of the orthorhombic system show one direction of perfect cleavage and some material is fibrous. The hardness is about 7.5 for the crystals and from 6 to 7 for the fibrous material. The SG is 3.23–3.24 and the RI for the alpha, beta and gamma rays is 1.653–1.661,1.657–1.662 and 1.672–1.683, biaxial positive with a birefringence of 0.020.Pleochroic colours, often distinct, are pale yellow to green; brown or greenish; dark brown or blue, sometimes violet. Absorption bands may be seen at 462, 441 and 410 nm. Sillimanite occasionally shows a red fluorescence which is apparently recorded for Myanmar specimens only.Sillimanite is found in high-grade metamorphic schists and gneisses,sometimes in pegmatites. Probably most gem sillimanite is found in gem gravels. Chief occurrences are in the Mogok Stone Tract, Myanmar,where violet–blue stones have been reported and in the Sri Lanka gem gravels, where a greyish green chatoyant material is obtained.Small-sized material similar to the Myanmar specimens is reported from Kenya. Chatoyant stones turn up from time to time. Sillimanite is one of the most difficult transparent gem materials to fashion.