DEFINITION OF CHERT

A fine-grained chemical sedimentary rock that

exhibits concoidal fracture and has been used for stone tools

such as arrowheads and knives by prehistoric peoples. It is

dense, typically has a dull to semivitreous luster, and has a

variety of colors based on types of impurities. The dark gray

variety is known as flint, whereas the bright red variety containing

iron oxide impurities is known as jasper. Most of the

silica in chert consists of very small microcrystalline or cryptocrystalline,

less than 30 micron interlocking quartz crystals,

but chert may also contain amorphous silica known as opal.

Chert may form as diagenetic (replacement) products in limestone,

often forming rows or football-shaped nodules. It also

forms bedded deposits that may form by accumulation of

siliceous tests (skeletons) of planktonic marine organisms, or

through chemical and biochemical reactions in seawater.

See also CHEMICAL SEDIMENT.

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