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.