Is a Zinc Iron sulfide with formula (Zn, Fe)S and hardness of 3.5-4
Surely one of the most beautiful cut gems of the world, despite his beauty, is too soft and fragile to wear and the material is rather brittle and easily scratched. Larger stone usually have some inclusions, as well as flows, so a completely and transparent gem is also considered rare. These stones are very bright with dispersion four times higher than diamond.
Comes from Spain, but the mine is close since many years, so rough material is difficult to obtain.