Diamond has many unequaled qualities. It is the hardest known substance, the best conductor of heat, has the highest melting point of any substance, and has the highest refractive index of any natural mineral.

It is number 10 on the Mohs scale. Diamond is 40 times harder than Corundum, which is number 9 on the Moh's scale.

Because of their heat conduction, Diamonds feels cold at or below room temperature. If heated, they remain hot for a long while after the heat source is removed.

The luster of Diamonds are incomparable. They exhibit great "fire", or brilliance, which gives them a shiny, freshly polished look. Rough Diamonds exhibit a greasy luster, but proper cutting give them a powerful adamantine luster. The only substances that can reach or excel the refractive index of Diamonds are synthetic substances (such as synthetic Rutile), but they are too soft to be used as gems.

There are different varieties of Diamond. The hardness and refractive index may slightly vary among specimens. Bort and Carbonado exhibit a slightly lower hardness than other Diamonds, and lack cleavage. Lonsdaleite (or Hexagonal Diamond), is a type of Diamond found with meteorites and is of extraterrestrial origin. It is scientifically a different mineral than Diamond, and believed to have formed when meteoric Graphite fell to earth. When this happened, great heat and stress transformed the Graphite into Diamond, but it retained Graphite's hexagonal crystal lattice.

Some interesting properties of Diamond:

Diamond is the hardest substance known
Diamond is the greatest conductor of heat
Diamond has the highest melting point of any substance (7362° F - 4090° C)
Diamond has the most dense atomical configuration
Diamond is transparent over the greatest number of wavelengths.