Fine Opal (SiO2·nH2O) is actually not technically a mineral but instead is a mineraloid. The reason being opal does not have a crystal structure that is required for all minerals. As noted in the chemical formula, opal is a hydrated form of silica or silicon dioxide. Opals can form in a variety of different colors depending on the environmental conditions during formation. The mineralogy of opal allows it to diffract light, causing it to shimmer various colors.