Opal gemstones. This mineral is a form of hydrous silicon oxide (silica, SiO2). It is named after the Indian word for precious stone. After it has been cut and polished, it has an iridescent sheen. This is as a result of its composition of microspheroids of silicon oxide that refract light. This is enhanced by reflections from tiny conchoidal fractures (crazing) that occur on exposure to the air. Opal never occurs in a crystalline form, but as small veins, globules and crusts. Several types occur: hazy blue, milky white, colourless and the highly prized red (fire opal) and black.