The vivid colours of corundum gem varieties, such as ruby and sapphire, arise primarily from elemental substitution in the Al site by transition metal elements. This mineral also has low dispersion so the value of the stones comes not from fire generated (as in diamond), but rather from the intensity of colours seen. What Colours can Corundum Have, How are These Colours Generated, and What Gem Varieties Result? Some varieties of corundum will fluoresce under short wave and long wave UV light if there is enough chromium in the crystal structure but little iron, which tends to quench any emitted energy. Red corundum is called ruby, blue corundum is called sapphire, and all other colours are called fancy sapphires.

A high density of ~4.0 g/cm 3 (most silicate minerals are ~2.6 g/cm 3) results in corundum occurring in secondary placer deposits and recoverable by panning methods, similar to how you would recover placer gold.Ĭorundum comes in all colours of the rainbow but is most commonly found as opaque crystals with dull colours.

It has no dominant cleavage and fractures in a conchoidal manner. It has a hardness of 9 on the Mohs scale, making it one of the most durable commercial gemstones. Lesson 17: Corundum Mineralogy and Gemology What is Corundum and What are its Basic Qualities?Ĭorundum is an aluminum oxide that commonly forms hexagonal barrel-shaped prisms that taper at both ends or as thin tabular hexagonal plates.

Pink sapphire price per carat