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Cryptocrystalline is a rock texture made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically[1][2] 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 felsic type such as felsites and rhyolites, may have a cryptocrystalline groundmass as distinguished from pure obsidian (felsic) or tachylyte (mafic), which are natural rock glasses. Onyx is also a cryptocrystalline. Agates such as the fairburn agate are also composed of cryptocrystalline silica.

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  1. ^ Dictionary of geological terms. Bates, Robert Latimer, 1912-, Jackson, Julia A., 1939-, American Geological Institute. (3rd Anchor books ed.). Garden City, N.Y.: Anchor Press/Doubleday. 1984. p. 120. ISBN 0385181000. OCLC 9412868.CS1 maint: others (link)
  2. ^ Clark, Roger. (2009). South Dakota's State Gemstone : [autographed copy] ; Fairburn Agate. Clark, Mary, Jane. (1st ed.). Appleton, WI: Siverwind Agates. ISBN 9780966464023. OCLC 698895703.