Chert is a microcrystalline or cryptocrystalline sedimentary rock made mostly of silicon dioxide (SiO2). It can form as nodules, concretions, and as layered deposits. Like other silica rocks and minerals, chert breaks with a conchoidal fracture, often producing very sharp edges. Native Americans took advantage of this fracture pattern and intentionally knapped chert to make arrowheads and other cutting tools and weapons. Since chert forms in sedimentary rock, it often can contain fossils as well as banded layers.
There are two main ways chert forms. In some cases, chert develops when microcrystals of silica grow in deposits of limestone or chalk. This occurs when dissolved silica is transported through sedimentary layers by groundwater. Large numbers of silicon dioxide microcrystals grow from the dissolved silica into irregularly-shaped nodules or concretions. If there is a lot of silica causing large number of nodules to form, the nodules can merge together to develop a contiguous layer of chert within the sedimentary rock. When chert develops from dissolved silica it is classified as a chemical sedimentary rock.
The other way chert forms is from biologic remains. Certain marine organisms contain silica in their exoskeletons or spicules, such as sponges, radiolarian, and diatoms. When these organisms die, their remains fall to the bottom of the oceans or shallow seas. The silica dissolves, recrystallizes, and develops into chert nodules or entire layers of chert.
Most chert is tan, cream color, or gray. When iron impurities are included within the nodules or layers, chert can also be red, green, or black. In some cases red chert, or chert with other colors, is classified as jasper. The term “flint” is used to describe varieties of chert that form in chalk formations, whereas chert usually forms in limestone formations. Some people make a distinction between “flint” and “chert” as a matter of quality – chert being lower quality than flint. Sometimes jasper is also considered a higher quality of chert.