Limestone has two origins: (1) biogenic precipitation from seawater, the primary agents being lime-secreting organisms and foraminifera; and (2) mechanical transport and deposition of preexisting limestones, forming clastic deposits. Travertine, tufa, caliche, chalk, sparite, and micrite are all varieties of limestone.

Limestone is usually a biological sedimentary rock, forming from the accumulation of shell, coral, algal, fecal, and other organic debris. It can also form by chemical sedimentary processes, such as the precipitation of calcium carbonate from lake or ocean water.

Limestone is an organic, sedimentary rock. This means it was formed from the remains of tiny shells and micro-skeletons deposited on the sea bed. Over the years this sediment was compressed to form solid rock. Limestone is formed in layers – called bedding planes. These bedding planes contain vertical cracks called joints.

Limestone (calcium carbonate CaCO3) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium.

Limestone is a type of rock that is made up of bits of animal shells. Over millions of years these shells collected on the ocean floor. As layers of shells and mud built up, the lower layers slowly hardened into limestone.