Gibbsite is an aluminium hydroxide mineral, chemical formula Al(OH)3. Its crystal system is monoclinic, the crystal habit is massive, with well-developed crystals being an exception. Concretions and smooth or encrusted botryoidal forms are common, with colour ranging from colourless to shades of grey, green and blue. Being a major constituent of bauxite, gibbsite can be indirectly considered an important ore of aluminium. Specimen is from Minas Gerais in Brazil, actual width 60 mm.