Opal

Opal is a hydrated silicon dioxide mineraloid, with the general formula SiO₂·nH₂O, which does not have a regular crystalline structure. It comes in various colors, including white, cream, yellow, orange, pink, blue, green, or black. Some opals exhibit a spectacular play of color called opalescence, produced by the diffraction of light within their internal structure. Notable varieties include precious opal, common opal, fire opal, and black opal. It primarily forms in fissures and cavities in rocks, through the gradual deposition of silica from water-rich solutions.