Al2O3, Aluminum Oxide, Corundum, Sapphire
Aluminum oxide is a ceramic compoundwith a hexagonal crystal lattice. The oxygen anions define a hexagonal close packed structure and the aluminum cations occupy 2/3 of the octahedral sites in the hcp lattice. Because of the high charge (3+) on the cations, the aluminum ions want to have the maximum spacing possible in the structure. The structural unit cell is shown in the diagram. Because of the empty Al-sites in the basal planes of the cell, the structure repeats itself every six layers, giving a c0 value of 1.299nm. 
From: Chiang, Birnie III, and Kingery, "Physical Ceramics," Wiley, MIT (1997)