Bismuth

Bismuth is a metallic element (Bi, 83). it has a relatively low melting point. native bismuth is relatively uncommon, but bismuth crystals are easy to grow and found in most rock and gem shops. 

Bismuth crystals have a very distinctive shape. They look like little castles and steps. bismuth crystals form rapidly over a short space of time. They row so rapidly that the edges of the crystal grow faster than the interior. When this occurs the resultant crystal is a skeletal crystal. Halite (table salt) forms very similar crystals in the right conditions.

The iridescent colours on the surfaces of the crystals is caused by bismuth oxide. The oxide forms different thicknesses, this creates an interference effect