Mineral spotlight: corundum

Better known from its colored varieties, ruby and sapphire, corundum is a highly appealing mineral for both decorative and industrial use. The bright, deep colors along with its rarity and high hardness has made it an excellent gemstone throughout human history. Ruby is the name given to the red variety of corundum, while sapphire refers to any non-red variety of corundum. Dark brown or black corundum is known as emery, and while most corundum is not gem grade, it can be used as an abrasive due to its hardness.

Corundum is an aluminum oxide mineral with the composition of Al2O3. It forms most commonly in granite pegmatite environments, where many other highly sought after minerals are found. Pegmatites are ancient magma bodies which have cooled over long periods of time to form igneous rocks with large crystals. Corundum can also be found in metamorphic environments, where minerals are changed under heat and pressure.

In Tanzania, deeply colored rubies can be found in a matrix of the bright green mineral zoisite. This combination is called anyolite, and makes for an impressive display of colors.

The mineral rutile (titanium dioxide) occasionally grows in line with the hexagonal crystallographic axis of corundum, and when cut and polished into a dome (called a cabochon), a star like structure can be seen when turned in the light, which contrasts beautifully with the dark color of the mineral. These are known as star rubies or sapphires, and this fantastic play with light is called “asteris,”, a highly desirable property that can also be seen in some garnets.

In addition, rubies are commonly fluorescent and will glow a bright pink-red under long and short wave ultraviolet light due to trivalent chromium impurities.

Come see our phenomenal ruby samples in the Dice Mineralogical Museum, located on the first floor of North Hall and open from 12:30 to 4 p.m. every Wednesday, Thursday and Friday.