What is "cryptocrystalline"

Cryptocrystalline \Cryp`to*crys"tal*line\ (-kr?s"tal-l?n), a. [Gr. krypto`s hidden + E. crystalline.] (Geol.) Indistinctly crystalline; -- applied to rocks and minerals, whose state of aggregation is so fine that no distinct particles are visible, even under the microscope.

a. (context petrology English) Whose crystalline nature only becomes apparent at high levels of magnification. n. (context petrology English) A rock of this kind.

Cryptocrystalline is a rock texture made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically in thin section by transmitted polarized light. Among the sedimentary rocks, chert and flint are cryptocrystalline. Carbonado, a form of diamond, is also cryptocrystalline. Volcanic rocks, especially of the acidic type such as felsites and rhyolites, may have a cryptocrystalline groundmass as distinguished from pure obsidian (acidic) or tachylyte (basic), which are natural rock glasses. Onyx is also a cryptocrystalline.