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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
Felsite is a very fine grained volcanic rock that may or may not contain larger crystals and light colored rock that typically requires petrographic examination or chemical analysis for more precise definition
From English feldspar and -ite
Durable Rock, Medium Hardness Rock
Fine Grained Rock, Opaque Rock