When someone lists the most famous gemstones such as
diamond, topaz, aquamarine, emerald and garnet, corundum
does not usually get mentioned. However, its two varieties
are sure to be on any list of gemstones. The red variety
of corundum is known as ruby and all the other colors
of corundum are known as sapphire.
Corundum is the second hardest natural mineral known
to science. The hardest mineral, diamond is still four
times harder than corundum. The hardness of corundum
can be partially attributed to the strong and short
oxygen-aluminum bonds. These bonds pull the oxygen and
aluminum atoms close together, making the crystal not
only hard but also quite dense for a mineral made up
of two relatively light elements.