Chert

Chert is a sedimentary rock consisting almost entirely of silica (SiO ₂), and can form in a variety of ways. Biochemical chert is formed when the siliceous skeletons of marine plankton are dissolved during diagenesis, with silica being precipitated from the resulting solution. Replacement chert forms when other material is replaced by silica, e.g. petrified wood forms when silica rich fluids percolate through dead wood and the silica precipitates to replace the wood. Chert can also form through direct precipitation from silica rich fluids, e.g. agate is formed by the precipitation of silica in voids within a rock. Chert has the general physical properties of quartz.

Texture - non-clastic.
Grain size - cryptocrystalline, cannot be seen except under very high magnification.
Hardness - hard.
Colour - all colours, dependent on impurities present when precipitated.
Clasts - none.
Other features - smooth to touch, glassy, exhibits conchoidal fracture.
Uses - mainly decorative; ancient cultures used chert for cutting tools, arrow heads etc.