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'''Corundum''' is a very hard mineral - [[Moh's hardness scale]] of 9, second only to a [[diamond]].  When comparing absolute hardness, a diamond is four times harder than corundum.<ref>http://www.galleries.com/minerals/hardness.htm</ref>  A clear and colorless synthetic corundum is often used for watch crystals to resist scratching as it can only be scratched by another corundum surface or a diamond.

'''Corundum''' is a very hard mineral - [[Hardness|Moh's hardness scale]] of 9, second only to a [[diamond]].  When comparing absolute hardness, a diamond is four times harder than corundum.<ref>http://www.galleries.com/minerals/hardness.htm</ref>  A clear and colorless synthetic corundum is often used for watch crystals to resist scratching as it can only be scratched by another corundum surface or a diamond.

Corundum is an aluminum oxide.  In perfect form, corundum is clear though various impurities will change the color of the mineral. When colored red, corundum is called a [[ruby]].  Other clear and translucent forms of corundum are called [[sapphire]]s.   

Corundum is an aluminum oxide.  In perfect form, corundum is clear though various impurities will change the color of the mineral. When colored red, corundum is called a [[ruby]].  Other clear and translucent forms of corundum are called [[sapphire]]s.