Bismuth

Bismuth is rarely found naturally in its elemental form (even less commonly than platinum), but lab-grown crystals are gaining in popularity for their unique geometric formations (hopper crystals) and phenomenal iridescence. Bismuth is a silver-white metal, but an oxide layer forms immediately when crystals make contact with air. This produces an array of colors similar to that of a soap bubble or oil on water. The different colors are dependent upon the thickness of the oxide layer, which determines how light is reflected off of the surface creating a striking rainbow effect.