In a typical fossil, the body form is retained, but the original molecules that made up the body have been replaced by some inorganic material, such as calcium carbonate (CaCO3) or silica (SiO2). The fossil feels like, and is, made of rock. It has been mineralised or petrified (literally, turned into rock).
A fossil may also be an imprint or impression of a living thing remaining in the fossilised mud of a long-gone age.
Some organisms fossilise well, others do not. The most common fossils are those left behind by organisms that produce hard materials. The hard, calcitic shells of molluscs (such as clams and snails) and of now-rare brachiopods (also known as lampshells) are examples. These sea-dwelling shellfish have produced many fossiliferous (that is, fossil-bearing) chalkylayers of limestone in the earth.
Soft-bodied organisms can fossilise in special circumstances: the Ediacaran biota is a good example.