When the texture of a
rock 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, the rock is said to be
cryptocrystalline. 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.