Chert

Chert, or HORNSTONE, a variety of quartz, always massive, not unlike flint, but more brittle, breaking with a splintery fracture. It is common in limestones of Paleozoic age, but occurs also in Mesozoic strata (Jurassic, Cretaceous), sometimes forms rocks, and often contains petrifications. It passes into common quartz and chalcedony, also into flint and flinty slate. Its colours are gray, white, red, yellow, green, or brown. The name Chert is sometimes limited to the finer varieties, and the coarser are called Hornstone.—The name Chert is very commonly given to the siliceous concretions which occur as nodules and layers in limestone rocks, much in the same way as flints in the chalk. When these materials exist to such an extent as to render the limestone useless for economical purposes, it is said to be 'cherty.'