A hopper crystal is a distinctive growth pattern that occurs in certain natural elements, including Bismuth, Galena, Gold, and Halite (salt). Hoppering occurs when electrical attraction is higher along the edges of the crystal, this causes faster growth at the edges than near the face centers. This attraction draws the mineral molecules more strongly than the interior sections of the crystal, thus the edges develop more quickly. This results in what appears to be a hollowed out step lattice formation, as if someone had removed interior sections of the individual crystals.
For Bismuth, 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. The spiral, stair-stepped structure of bismuth crystals is the result of a higher growth rate around the outside edges than on the inside edges.