Diamond has many unequaled qualities and is very unique among minerals. It is the hardest known substance, it is the greatest conductor of heat, it has the highest melting point of any substance (7362° F or 4090° C), and it has the highest refractive index of any natural mineral. Diamond is number 10 on the Mohs scale, and is 40 times harder than Corundum, which is number 9 on the Moh's scale. It also has the most dense atomical configuration of any mineral, and is transparent over the greatest number of wavelengths. Because of heat conduction, Diamonds are cold to the feel at or below room temperature. When heated, a Diamond will remain hot long after the heat source is removed.
The luster of Diamond is excellent. Diamond exhibits great "fire" and brilliance, which gives it a shiny, freshly polished look. Rough Diamonds exhibit a greasy luster, but proper cutting give them a powerful adamantine luster. Only synthetic substances and a few minor gemstones can reach or excel the refractive index of Diamonds.
The hardness and refractive index may slightly vary among Diamond specimens. Bort and Carbonado exhibit a slightly lower hardnes than other Diamonds, and lack cleavage. Lonsdaleite (also known as 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.