Opal is a hydrated amorphous variety of silica, chemical formula SiO2.nH2O) It is classified as a mineraloid, because of its amorphous habitat, as opposed to normal silica forms that are crystalline. Fire opal is a variety of the mineral opal characterized by fire-like yellow to orange red hues. It is a highly prized semi precious stone used in jewelry. Actual width is 45 mm.