Clay is plastic semi-bound sediment formed by diagenesis (binding) of sludge, pelitic material transported by water and deposited aquatic environment. In addition to clay which forms by transporting and depositing sludge material, there are also those which form and remain at the place of decomposition of the primary material. These are so-called residual or sedimentary clays.

This pelitic material can be bonded either by drying or squeezing water under the pressure of the upper layers. Clay represents the middle stage in the diagenesis of sludgy material. Under the pressure, or very intensive drying, this material loses its plasticity and turns into a solid layered rock called argil.

Clay and argil are very widespread sedimentary rocks. They consist of clay minerals and various impurities.

Among the clay minerals the most important are kaolinite, hydrolysis (illite), montmorillonite, and other aluminum silicates. We consider quartz grains, and very rarely, zircon, apatite, garnet and others, as secondary ingredients of clay. Clays often contain iron hydroxides as impurities, which pigment the rock reddish, yellow or dark red. They often contain other substances (especially bitumen), which again give them a dark grey or even black color. Small amounts of manganese color the rock greenish.

Clays can be dominated by one of the clay minerals, and then they are monomineral clays, or several minerals are represented in approximate quantities, then we call them polymineral.