Ruby crystal structure, illustration. A ruby is a derivative of corundum (Al2O3). The oxygen atoms (smaller) are arranged around the aluminium atoms (larger) in Al(III)O6 octahedra. Corundum, among the hardest of natural minerals, is colourless when pure, but various transition metal ions often contaminate a small fraction of the aluminium sites in the crystal. When the impurity is Cr(III), the substance is a ruby.