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Chert is a hard, dark, opaque sedimentary rock which is composed of silica with an amorphous fine-grained texture
From flint-like quartz, 1670s, of unknown origin- a local term, which has been taken into geological use
Fine Grained Rock, Opaque Rock
Gabbro is an intrusive igneous rock which is chemically equivalent to plutonic Basalt
Christian Leopold von Buch
From Latin glaber bare, smooth, bald
Coarse Grained Rock, Opaque Rock