corundum
/kəˈrʌndəm/
noun
- A very hard mineral, often used as an abrasive or as a gemstone (ruby or sapphire).
- Corundum is one of the hardest natural materials, second only to diamond.
- Jewelers value certain types of corundum, like ruby and sapphire, for their beauty.
- The factory uses corundum powder to polish metal surfaces.