Gibbsite. The mineral gibbsite is an aluminium hydroxide, simplified formula Al(OH)3, crystal system monoclinic. It is usually found in metamorphous, aluminous or phosphatic rocks. An alternative but less used name for it is hydrargillite. The habitat is massive, with well-developed crystals being an exception. Concretions and smooth or encrusted botryoidal forms are common, ranging from colourless to shades of grey, green and blue. Gibbsite is an important ore of aluminium, and that mostly as a constituent of bauxite. This sample is from Hot Springs in Arkansas, USA. Actual width is 65 mm.