A mineral: SiO₂. It is a high-temperature polymorph of quartz and tridymite, and occurs as white octahedrons in the cavities of the fine-grained groundmasses of acidic volcanic rocks. Cristobalite is stable only above 1470 degrees C; it has a tetragonal structure (alpha-cristobalite) at low temperatures and an isometric structure (beta-cristobalite) at higher temperatures. Compare: tridymite