Unusual group of bismuth crystals Bismuth is a brittle metal with a tone often white, pink silver, a stain iridescent oxide showing many colors from yellow to blue. The spiral structure and staggered bismuth crystals is the result of a higher growth rate around the outer edges than at the inner edges. Variations in the thickness of the oxide layer formed on the surface of the glass make different light wavelengths interfere reflection, thus showing a rainbow color