Elastic sedimentary rock consisting of sand with a very fine grain and of cement in which hydrated silicate of aluminum (Kaolin) is abundant. It presents particular characteristics of plasticity and has the capacity to absorb water. On the basis of the amount of kaolin contained, fat clay and thin clay are distinguished. The colour varies from clear gray to greenish and from yellow to reddish (this last case is due to a strong presence of iron oxide). Other than being used as a pigment, clay is used in the brick and ceramic industry.