The group name for cryptocrystalline varieties of silica composed of minute crystals of quartz with submicroscopic pores and with composition ranging from SiO2 to SiO2.nH2O and including the minerals agate, chert, opal, onyx, jasper, and flint; sp. gr. 2.50–2.67; habits variable from stalactitic to massive; commonly white, greyish-white, or grey, and occasionally yellow.