Sillimanite, related to both andalusite and Kyanite, it all belongs to the group of polimorphs aluminium silicates. Is named in Benjamin Silliman’s honour that was chemistry professor at Yale University.
Chemical composition: Al2 SiO5.
Crystal system: Ortorombic.
Refactive index: 1659-1680.
Birrefringence: 0.020 a 0.023.
Origin: Madagascar, Sri Lanka, India.
It’s easy to find in masive crystals, ussually known as fibrolite. This kind with a good work of cabochon cutting can display a chatoyancy or cat’s eye effect due to it’s internal fibrous structure. These crystals have a hardness of 6.5-7 on the Mohs scale.
It occurs in colours like colorless, grey-white, yellow-greenish, bluish, brownish. It’s not commonly founded in isolated and transparent crystals, in fact are extremaly rares the specimens in gem quality and good colour. For this reason the gems with vivid and beautiful colours are very valuable on the market. These crystal has a hardness of 7.5 on the Mohs.
Transparent variety specially the good colour gems has a acussed pleochroism; for this reason this gems due to it’s relative high birrefringence is easy to notice a dipersion or fire that means are valuable stones to be setted on jewellery pieces.