Formation
(1) Through the crystallization of amorphous silica from supersaturated, hydrothermal, or meteoric waters as a result of cooling or evaporation, usually within cavities in volcanic rocks or sediments.
(2) Through biogenic accumulation and diagenesis of skeletal substances from siliceous organisms (e.g., diatoms).
(3) As pseudomorphic replacement of organic or mineral structures in weathering zones.
