Dendrite

Dendrites look like fossils, but they are not the remains of living things. Dendrites (sometimes called dendrolites) are growths made of dark manganese minerals, usually psilomelane, that form in a branching (dendritic or arborescent) pattern. That is, they display the dendritic mineral habit.

Dendrites are found between the layers of stratified rocks, as in this Colorado sandstone, and within bodies of agate. Dendritic or moss agate is a very popular gemstone.

Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society