These beautiful man-made crystals float to the top of a super-cooled bismuth melt and form within minutes. Bismuth is one of the few materials that have a greater density as a liquid than as a solid. Only a few other materials such as water, gallium, and germanium, exhibit this property. The trigonal crystal structure accounts for the cubic formations. The iridescent rainbows of colors observed are caused by light scattering off thin layers of bismuth oxide formed when the surface of the hot bismuth reacts with the air. Specimens range from 12 to 17g with a density of 9.81 g/cc. Great for teaching about crystals, states of matter, or even as a gift!