Kyanite

Kyanite, also known as disthene, is interesting for two reasons. Its structure is so grained in character that the hardness varies in the same stone from 5 to 7 on Mohs’s scale; it can therefore be scratched by a knife in some directions, but not in others ([p. 79]). It has the same chemical composition as andalusite, both being silicates of aluminium corresponding to the formula Al₂SiO₅, but possesses very different physical characters, a fact which shows how large a share the molecular grouping has in determining the aspect of crystallized substances. It is biaxial with small negative double refraction, the least and greatest of the refractive indices being 1·72 and 1·73 respectively; the specific gravity is 3·61. It occurs in sky-blue prismatic crystals, whitish at the edges, in schist near St. Gothard, Switzerland. It is seldom cut.

Kyanite is derived from its colour, κύανος blue, and disthene, from its variable hardness, δίς, twice, and σθένος, strong.