A cryptocrystal is a rock whose texture is so finely crystalline—that is, made up of such minute crystals—that its crystalline nature is only vaguely revealed even in a thin section by transmitted polarized light. Among the sedimentary rocks, chert and flint are cryptocrystalline. Lava flows, especially of the acidic type such as felsites and rhyolites, may have a cryptocrystalline ground mass as distinguished from pure obsidian (acidic) or tachylite (basic), which are natural rock glasses.