Magnesium aluminum spinel (MgAl₂O₄) is a face-centered cubic material, the prototype for the spinel structural family, which includes binary oxides, sulfides, and selenides. The space group is Fd3m, unit cell length is 0.8084 nm, and the unit cell has eight formula units. The positions of the ions in the unit cell have been determined by both X-ray and neutron diffraction techniques. There is an inverse spinel structure, wherein the aluminum ions occupy both Td and D3d sites, and the magnesium ions occupy some of the D3d sites. The occurrence of the normal, inverse, or in-between structure depends on the ionic size, method of preparation, and composition. Natural magnesium aluminum spinel has the normal spinel structure; however synthetic forms of spinel prepared at high temperature have the inverse structure. The composition of magnesium aluminum spinel varies from near stoichiometric to highly alumina-rich. Optical-quality spinel can be synthetically made with a variety of techniques including: growth using the Czochralski method, the gradient furnace technique, hot pressing of powder, press forging, flame fusion, and fusion casting.