corundum

This term refers to a specific mineral species characterized by an exceptionally high Mohs hardness, second only to diamond. It is most frequently encountered in gemology, where its most prized varieties are known as rubies and sapphires. The word carries a technical, scientific connotation, typically used in geological reports or mineralogical catalogs rather than casual conversation.

As a mass noun representing a chemical compound and mineral species, it is treated as uncountable. It does not have a plural form in standard scientific usage; instead, practitioners refer to specimens, crystals, or grains of corundum to denote individual units.