Sillimanite (also known as Fibrolite) is found in various colours including green, yellow, brown, blue, white and even black. It is a ‘polymorph’ with two other minerals, Kyanite and Andalusite, which simply means that it shares the same chemistry but has a different crystal structure.
Like Kyanite, this precious stone has a brittleness and directional cleavage that makes it difficult to facet. The scarcity of Sillimanite, combined with the difficulty in cutting it, adds incredible value to this gem once it has been successfully faceted.
The gem is a complete Jekyll and Hyde; it can be discovered as a truly transparent gem, or completely opaque with a waxy lustre. The gem’s name was given in memory of geologist Benjamin Silliman.
It appears in deposits in several countries including Tanzania, South Africa, Korea, India, Madagascar, England and the USA. The rare transparent variety is mined in the Mogok Valley, Burma, where the deposit has also yielded some extremely rare violet-blue gems.