+Corundum is a crystalline form of [Aluminum Oxide](/wiki/aluminum_oxide), renowned for its extreme [Hardness](/wiki/hardness). Its most celebrated gem varieties are the vibrant red [Ruby](/wiki/ruby) and the deep blue [Sapphire](/wiki/sapphire).
+## See also
+- [Mineral](/wiki/mineral)
+- [Gemstone](/wiki/gemstone)
+- [Mohs scale](/wiki/mohs_scale)