Pseudofossils.

Pseudo or false fossil, dendrites of Manganese dioxide. Source: Museum Victoria.

A variety of mineral formations look remarkably like fossils, but are not fossils at all. They are known as 'false' or 'pseudo' fossils, and include concretions, dendrites and other crystal growths. All are formed by inorganic (non-living) processes.

Concretions are irregularly shaped mineral nodules that form around a nucleus of some sort within sedimentary rock. Their composition differs from that of the surrounding rock. Concretions made up of pyrite or goethite, in particular, may sometimes resemble fossils.

Dendrites look as though they are plant fossils of some kind, as they have a branched, fern or tree-like appearance, and occur in sedimentary rock. They are actually crystalline growths, often of the manganese mineral pyrolusite. Crystals of various other minerals can also, with a little imagination, be confused with fossils. Examples include malachite, which occasionally has a bulbous appearance, and imprints of gypsum crystals in mud, which may resemble animal shapes.