Opals are microcrystalline or noncrystalline forms (Graetsch, 1994) of SiO2 or SiO2 . nH2O. The H2O is not structurally required. Opal-C (synonym: lussatine) refers to a form with a disordered cristobalite (C) structure, opal-CT (synonym: lussatite, common opal) refers to disordered cristobalite/tridymite intergrowths (CT), and opal-AG (synonym: precious opal) is comprised of cubic- or hexagonal-stacking of closest-packed silica spheres (amorphous spheres of equal size, ~0.0003 mm), where A = amorphous, G = gel-like. Precious opal exhibits opalescence or play of colors in reflected, white light. Opal-AG (synonym: potch opal) lacks opalescence because of disorder in the stacking of planes of the closest-packed spheres. Opal-AN (synonym: hyalite) occurs as botyroidal masses, where N = network or glass-like forms.