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 approximately 4 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 hardness and refractive index may slightly vary among Diamond specimens. Bort and Carbonado exhibit a slightly lower hardness 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.