Pseudofossils are inorganic objects, markings, or impressions that might be mistaken for fossils. Pseudofossils may be misleading, as some types of mineral deposits can mimic lifeforms by forming what appear to be highly detailed or organized structures. One common example is when manganese oxides crystallize with a characteristic treelike or dendritic pattern along a rock fracture. The formation of frost dendrites on a window is another common example of this crystal growth. Concretions are sometimes thought to be fossils, and occasionally one contains a fossil, but are generally not fossils themselves. Chert or flint nodules in limestone can often take forms that resemble fossils.

Pyrite disks or spindles are sometimes mistaken for fossils of sand dollars or other forms (see marcasite). Cracks, bumps, gas bubbles, and such can be difficult to distinguish from true fossils. Debates about whether specific forms are pseudo or true fossils can be lengthy and difficult. For example, Eozoon is a complex lumpy form found in some rocks that was long thought to be a fossil algae until "specimens" were identified in recent lavas.

A marcasite (pyrite) "sand dollar"

Pseudofossil hoaxes

Perhaps the best known pseudofossil was Piltdown Man, which fooled the famous philosopher Pierre Teilhard de Chardin and the author Arthur Conan Doyle.

Pseudofossils may be manufactured by individuals who hope to sell them to an unwary buyer, but in the case of Piltdown Man, mischief was clearly at work.