However, if any iron is present it will usually absorb the red fluorescence from UV light. This means that UV energy from normal light is accepted into the crystal and then re-emitted at a lower energy level - conveniently in the red region, thus amplifying the intensity of red in ruby under daylight conditions.

When Cr is introduced into corundum it makes the mineral fluorescent under UV light. The red region of the electromagnetic spectrum (~650 nm) does not have very much absorption at all and results in all colours but red being blocked by ruby.īut there is another trick up ruby's sleeve that makes its red almost jump out at the observer. When Cr substitutes for Al, wide absorption bands are generated in the violet (~450 nm) and green-yellow (~500 nm) ranges, as well as overlap a bit into the blue region. There is no official cutoff for the amount of Cr needed for ruby, but usually rubies will have up to ~1 wt% of Cr 2O 3. The most common cations to substitute are Fe +2, Fe +3, Ti +4, Cr +3, and V +3.Ī continuum of colour saturation exists between pink sapphire and ruby that is correlated with trace amounts of Cr. 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.

Yellow sapphire and ruby