Chert is the name given to rocks formed from very fine-grained sediments, which are then submitted to pressure, which causes them to become chert rocks. Chert may range in sediment construction, and may include limestone as part of its composition. So rock layers where one experiences alternating layers of chert and limestone may very well be composed of the same materials, but experience different levels of pressure and existing environmental conditions at the time the layers of sediment were created. Chert can range in color from white, brown, red, green, and different color varieties. Chert may also be termed "washbed" gravel, commonly found in streams, rivers, and such bodies of fresh running water. Chert is used widely in construction, as gravel for cement mix, and as a paving material for driveways.