Chert

Sedimentary

Chert is a hard, micro-crystalline chemical sedimentary rock consisting mainly of the mineral quartz (SiO₂). Chert has a conchoidal fracture pattern (like glass). Chert may sometimes be a product of organic precipitation of shells from microscopic organisms called radiolaria falling to the sea-floor. Layered cherts called "ribbon chert" are found in the Franciscan Complex in the Marin Headlands — you may be able to see individual shells if you look with a hand lens. A dark variety of chert is called flint.