Corundum crystal structure, molecular model. Corundum (aluminium oxide, Al2O3) is the compound that forms rubies (when red) and sapphires (when blue or any other colour). In this model, aluminium ions are pink and oxygen ions are grey. The unit cell (repeating structure) of the crystal is outlined in the blue box. The oxygen ions have a hexagonal close packed (HCP) arrangement, with the aluminium atoms occupying some of the spaces between them. This makes corundum very dense and very hard. Non-gem quality corundum is often used as an abrasive in industry.