Definitions for silica minerals were presented, based on discussions held during a workshop on the subject. The definitions were designed to include sufficient description of the chemical and physical properties to distinguish a specimen from other species, to contain commonly available methods of property measurement, and to describe species with a likelihood of human exposure. Cristobalite (14464461), quartz (14808607), and tridymite (15468323) were described according to x-ray crystallographic data, and the interplanar spacing, relative intensity, source, and elemental analysis were presented. Diatomite was characterized as an x-ray amorphous silica mineral occurring as skeletons of diatoms visible by optical microscopy. Fumed silica (7631869), silica gel (7699414), and silica glass were also defined as amorphous solids, and the distinguishing characteristics of each were presented. Silica mineral was defined as a natural or synthetic crystalline or noncrystalline solid having a chemical composition of 90 percent or greater silicon-dioxide (7631869). Crystallinity, particle size, and x-ray diffraction were also defined. The author concludes that the silica minerals should be periodically redefined, and that future workshops should have access to more extensive information on biological mechanisms.