chert

Chert is a fine-grained siliceous rock, commonly found as fist-sized nodules embedded in a limestone matrix. When you break it, it fractures smoothly in a conchoidal pattern -- in rounded flakes. It was used by people all over the world to make stone tools.

Flint, jasper, and agate are all varieties of chert. Plain brownish chert is simply called chert. Flint is dark grey or black. Jasper and agate are used as semiprecious stones for their banded or flowery crystalline patterns.

Chert is Silicone Dioxide, SiO2, and is a member of the Quartz family. It is found among limestone formations as well as in layers throughout banded iron formations.

Chert is a sedimentary rock, and is often formed by the the fosilized depostions of deep marine creatures, called siliceous ooze. The creatures that create Chert are found deep underwater, near the ocean floor where sea water is acidic and would eat away at the calcium carbonate that comprises the shells of shallow sea water animals. Most commonly, these animals are deep sea sponges.

Chert has been used for stone weapons and tools such as arrowheads and knives since ancient times. It is especially useful for these purposes due to its hardness (~7 on the Mohs hardness scale) and its conchoidal fracture, which leave it with very sharp edges.

Chert (?), n. [Ir. ceart stone, perh. akin to E. crag.] Min.

An impure, massive, flintlike quartz or hornstone, of a dull color.

 

© Webster 1913.