It is a hard, extremely dense, dull to semiglossy sedimentary rock (microcrystalline to cryptocrystalline quartz). Chert consists predominantly of silica, with occasional impurities such as calcite, iron oxides, clay minerals, and the organic remains of marine organisms made of silica, amounting to about 10 percent. Because it is so finely crystalline, a characteristic of chert is its conchoidal fracture; thus, it breaks like glass into smooth, curved flakes. It may be white or one of various shades of gray, green, pink, red, yellow, brown, or black. Chert (Flint), occurs principally as nodules or concretions in limestone, dolomites, and chalk beds. Sometimes, however, it forms a bedded or layered deposit or a thin wedgelike discontinuous layer; such beds are commonly associated with volcanic deposition. Some are made up largely of spines and shells of silica secreted by microscopic organisms such as diatoms, radiolarians, and sponges,or of their partly dissolved and reprecipitated remains. Other cherts are of inorganic origin. Some precipitated around hot springs rich in silica, others formed when silica bearing solutions replaced wood, limestone, shale, or other materials, and some are associated with volcanic activity.