Boehmite or Böhmite is an aluminium oxide hydroxide (AlO(OH)) mineral, a component of the aluminium ore bauxite. It is dimorphous with diaspore. It crystallizes in the orthorhombic dipyramidal system and is typically massive in habit. It is white with tints of yellow, green, brown or red due to impurities. It has a vitreous to pearly luster, a Mohs hardness of 3 to 3.5 and a specific gravity of 3.00 to 3.07. It is colorless in thin section, optically biaxial positive with refractive indices of n? = 1.644 - 1.648, n? = 1.654 - 1.657 and n? = 1.661 - 1.668.
Boehmite occurs in tropical laterites and bauxites developed on alumino-silicate bedrock. It also occurs as a hydrothermal alteration product of corundum and nepheline.
It was first described in 1927 and named for German geologist Johann Böhm (1857–1938).