Corundum

Corundum is crystalline aluminum oxide (Al2O3), one of the hardest natural minerals on Earth (9 on the Mohs scale, second only to diamond) and the parent species of two of the world's most coveted gemstones: red ruby (colored by chromium) and blue-to-fancy-colored sapphire (colored by iron and titanium). Trace element substitutions produce a remarkable color range, while trapiche and star (asteriated) varieties, caused by rutile silk inclusions, are especially prized by collectors.

Typical gem-quality corundum comes from classic alluvial and metamorphic deposits such as Mogok and Mong Hsu in Myanmar, Ratnapura in Sri Lanka, Kashmir, and Ilakaka in Madagascar. Exceptional collector specimens include the small but flawless, untreated blue crystals of Yogo Gulch, Montana, prized for their uniform saturation and natural luster, and the historic hexagonal rubies of the Cowee Valley in North Carolina. Non-gem, rock-forming corundum defines the famous emery deposits of Naxos, Greece, and the corundum-syenite belts of Ontario, Canada, while opaque rubies in green zoisite matrix come from Longido and Winza in Tanzania, and Jegdalek, Afghanistan.

Collectors seek natural corundum for its unmatched hardness, saturated color, and crystallographic beauty, from barrel-shaped sapphire crystals to tabular Yogo gems and classic hexagonal ruby prisms. Untreated, well-formed specimens from historic type localities remain among the most desirable pieces in any hard-mineral or gemstone collection.