Aluminum oxide occurs in two different forms, the crystalline polymorphic phase α-Al₂O₃ as well as the metastable cubic γ-Al₂O₃. γ-Aluminum oxide is turned into α-Al₂O₃ when heated. Crystalline α-aluminum oxide is often used for ceramic insulators or as abrasives and has a Mohs hardness of 9 to 9.5. Due to this it is not economically feasible to grind crystalline aluminum oxide on Bauermeister mills. The main application for γ-Aluminum oxide, also known as clay or alumina, is as a starting material for ceramic products or in aluminum production. It is a softer, hygroscopic, white powder that is often used in cosmetics, medicine and pigments for colors. For these applications a high surface area is essential, which can be achieved by grinding the alumina to the desired particle size on a Bauermeister Gap Mill or Universal Mill.