Colemanite (Ca₂B₆O₁₁·5H₂O) is a borate mineral found in evaporite deposits. Colemanite is a secondary mineral that forms when borax is deposited, and then regularly covered with boron containing water. The boron remains after the water evaporates, and the boron and borax eventually form other minerals as for example Colemanite.

It was named after William T. Coleman (1824-1893), owner of the mine where it was first found.