Corundum is a crystalline form of aluminium oxide (Al2O3) typically containing traces of iron, titanium, vanadium, and chromium.

It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure.

Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present.

[6] In addition to its hardness, corundum has a density of 4.02 g/cm3 (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.

[13][14] The Verneuil process allows the production of flawless single-crystal sapphire and ruby gems of much larger size than normally found in nature.

Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market at a fraction of the cost of natural stones.

[18][19] However, its production is energy-intensive, contributing to carbon emissions if fossil fuels are used, and involves chemicals that can pose risks.

[20] Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), and laser components.

The drop with respect to load varies with the crystallographic plane due to the difference in crack resistance and propagation between directions.