The number of Solid phases of Alumina described in existence will be more than 25 and actually speaking it is doubtful whether all really exist. The list includes Amorphous Alumina, Hydrous Alumina, Anhydrous Alumina, Crystalline hydroxides and Oxides. Some are found in nature while others are artificially developed.
The pure forms found in nature are corundum, emery, sapphire and ruby. These forms contain alpha phase Alumina. The other natural minerals available and reported are Diaspore, Gibbsite and Bayerite.
The spinal structure Alumina is called Gama Alumina and other faces available are Eta, Theta, Kappa and Chi and they are differ from each other in crystal structure.
