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A rock is said to be cryptocrystalline when its 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.