When a pool of molten bismuth solidifies it tends to form crystals like those shown in these pictures. What gives these crystals their characteristic hoppered shapes is that there is more rapid growth at the edges of each face than at the center. The interior edges of a hoppered crystal appear to be a series of smaller and smaller stepped down miniature versions of the original crystal. The variations in the thickness of the oxide layer that forms on the surface of the crystal causes different wavelengths of light to interfere upon reflection, thus displaying a rainbow of colors.
Sash Segal: behance