Dendritic pyrolusite

Manganese dendrites.

Dendritic pyrolusite is a variety of pyrolusite, a mineral composed primarily of manganese dioxide (MnO2). It is characterized by its branched formations, similar to ferns or tree branches. These dendritic forms are caused by the precipitation of manganese minerals from aqueous solutions, often filling fractures or discontinuity planes in rocks.

Although pyrolusite is a manganese mineral, it can sometimes contain iron in its composition, which can influence its color and appearance.