1. n. 
A sedimentary rock and a variety of quartz made of extremely fine-grained, or cryptocrystalline, silica, also called chalcedony. The silica might be of organic origin, such as from the internal structures of sponges called spicules, or inorganic origin, such as precipitation from solution. The latter results in the formation of flint. Chert can form beds but is more common as nodules in carbonate rocks.
Alternate Form: cherty