Chert is a type of sedimentary rock that is composed of microcrystalline quartz. It is typically hard, dense, and often has a conchoidal fracture. Chert can form through the deposition of silica-rich sediments, such as from the remains of marine organisms or from the precipitation of dissolved silica in groundwater. It is commonly found in limestone and dolomite formations. Coal, conglomerate, and sandstone are all different types of sedimentary rocks, but they are not composed primarily of chert.