1. a very hard mineral composed of aluminum oxide, used as an abrasive and in making gemstones such as rubies and sapphires
Usage: mineralogy

Examples

• Corundum is one of the hardest naturally occurring minerals.
• The jeweler explained that rubies and sapphires are both varieties of corundum.
• Industrial corundum is used for grinding and polishing metals.
• The mineral corundum ranks 9 on the Mohs hardness scale.
• Synthetic corundum is often used in watchmaking for bearings.
• The red color in ruby comes from chromium impurities in corundum.