Chert

chert: see flint.

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chert [chərt]

(petrology)

A hard, dense, sedimentary rock composed of fine-grained silica, characterized by a semivitreous to dull luster and a splintery to conchoidal fracture; commonly gray, black, reddish brown, or green. Also known as hornstone; phthanite.

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chert

A very fine-grained dense rock consisting of chalcedony or opal, often with some quartz, and sometimes with calcite, iron oxide, organic matter, or other impurities; has a homogeneous texture and white, gray, or black color; some of its constituents may react with cement alkalies and therefore may be undesirable as concrete aggregate for exposed concrete in northern climates.

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Chert 

a mineral formation consisting of crystalline and amorphous silica (opal, chalcedony, or quartz). Chert is widespread in nature in the form of concretions, nodules, and seams that either lie parallel with the enclosing limestone and chalk deposits or cut across bedding. It is formed either by diagenesis of deposits, by catagenesis of rocks, or by weathering. It has a hardness of 7 on the mineralogical scale and breaks with a conchoidal fracture. Because of its ability to form a sharp cutting edge when chipped, chert (flint) was used by man from the most ancient times to the Iron Age to make stone tools.