Single-crystal aluminum oxide (sapphire – Al2O3) is a transparent ceramic. Sapphire's crystal structure is rhombohedral and thus its properties are anisotropic, varying with crystallographic orientation. Transparent alumina is currently one of the most mature transparent ceramics from a production and application perspective, and is available from several manufacturers. But the cost is high due to the processing temperature involved, as well as machining costs to cut parts out of single crystal boules. It also has a very high mechanical strength – but that is dependent on the surface finish.