The only gemstones higher on the Mohs hardness scale than corundum are moissanite (at 9.25) and diamond (at 10). Corundum is also unusual for its density–unusually high for a transparent material–making it ideal for a range of uses previously restricted to glass and leaded crystal.
Tiny shaped pieces of corundum were used in Swiss watches during the mid-1800s, as their durability and wear-resistance made them ideal for long-term use. In this use, they were called “jewel bearings”
Noteworthy deposits of corundum occur in Myanmar, Thailand, Madagascar, Sri Lanka and India. Other sources are Afghanistan, Vietnam, Greenland and multiple sources in the United States, including the states of Montana, Colorado, California and North Carolina.