Chert. Credit: U.S. Geological Survey.
Chert is a variety of quartz, always massive, not unlike flint, but more brittle, breaking with a splintery fracture. It is common in limestones of the Paleozoic era, but also occurs in Mesozoic strata (Jurassic, Cretaceous), sometimes forms rocks, and often contains petrifactions. It passes into common quartz and chalcedony, also into flint and flinty slate. Its colors are gray, white, red, yellow, green, or brown.
The name chert is sometimes limited to the finer varieties, and the coarser are called hornstone. Also, 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 chalk. When these materials exist to such an extent as to render limestone useless for economical purposes, it is said to be "cherty".