Gahnite

Gahnite consists of an aluminate of zinc, which crystallises in the regular system. usually in octahedra, resembling the spinels (q.v.). It varies in colour from green to blue-black or brown; has a specific gravity of 7.8, and hardness 4.5. It frequently contains, besides its chief constituents, iron or manganese and sometimes magnesium. It occurs in a few localities in Sweden, Bavaria, and America, but not very plentifully.