Bismuth (element #83 on the periodic table) forms beautifully colored and geometrically intricate hopper crystals. The crystal's eye-catching array of colors results from the formation of a thin oxide layer on its surface. The colors on Bismuth crystals arise in a similar fashion to those on a soap bubble or thin film of oil on water in which light reflecting off the top and bottom of the film produce interference maxima of a particular color depending on the film’s thickness. A thin layer of Bismuth Oxide on the otherwise pure Bismuth crystal causes light of certain wavelengths to interfere constructively upon reflection giving rise to the color seen on the surface. Due to variations in the thickness of the oxide layer, the crystal is not one solid color but rather is a rainbow of colors corresponding to the wavelengths (and colors) of light which interfere constructively at each location.