Ruby

A red, gem variety of corundum. The red colour is caused by minor amounts of trivalent Cr replacing Al in the crystal structure.

In traditional gemmological terms, ruby has to be blood-red and of clear, facetable quality to justify the name. However, in wider usage, any corundum with a red or reddish colour has attracted the name 'ruby', and this name is usually applied in this way by mineral collectors.

Pink corundum is typically referred to in gemmological terminology as pink sapphire rather than ruby.

Pure Cr2O3 occurs in nature as the mineral eskolaite which is green.

The dehydration of diaspore in corundum occurs between 525 and 550 °C, whereas goethite transforms to hematite between 300 and 325 °C. As both diaspore and goethite may be present as inclusions in rubies, sapphires, and other corundum varieties (e.g., pink sapphires, padparadscha), these dehydration reactions and phase transformations can be considered important criteria to separate unheated from heated stones. Specifically, in cases in which other methods (e.g., microscopy, FTIR) are unsuccessful.