Bismuth (Bi) is a naturally occurring element with an atomic number of 83, chemically, it resembles arsenic and antimony.
As you can see in the image, bismuth crystals are quite obviously angular- this is because the edges of the rhombohedral crystal structure are more energetically favourable positions than the interior of the structure. As a result, molecules continually ‘build on’ the edges but don’t fill in the centre when crystallising. The higher rate of growth on the edges forms a crystal which appears to be partially hollowed out in a rectangular-spiral stair step design.
Another obvious characteristic of bismuth crystals is their beautiful colours. This is the result of surface oxidation and consequently thin film interference. The surface of the crystal oxidises to bismuth oxide in extremely thin layers. Each of these layers causes light of certain wavelengths to interfere with each other upon reflection giving rise to the colour seen on the surface. Due to variations in the thickness of the oxide layer, the crystal is not one solid colour but rather a rainbow of colours corresponding to the wavelengths of light which arise at each location. Thicker layers produce reds and greens, and thinner ones blue and violets.
Interesting note: thin-film interference is also responsible for the dazzling colours in a bubble and the rainbows in an oil-spill. Via Facebook.