Diamond is the hardest naturally occurring mineral, topping Mohs' Scale of Hardness with a relative hardness value of 10.

Diamond is a polymorph of the element carbon, and graphite is another. While the two share the same chemistry, C (elemental carbon), they have very different structures and properties. Diamond is hard, graphite is soft (the "lead" of a pencil). Diamond is an excellent electrical insulator, graphite is a good conductor of electricity. Diamond is the ultimate abrasive (its most important use), graphite is a very good lubricant. Diamond is transparent, graphite is opaque. Diamond crystallizes in the isometric system, graphite crystallizes in the hexagonal system. However, at surface temperatures and pressures graphite is the stable form of carbon. In fact, all diamonds at or near the surface of the Earth are currently undergoing a transformation into graphite, although this reaction is extremely slow.

Facts about diamond:

  • Diamond is transparent over a larger range of wavelengths than any other substance, from the ultra-violet into the far infra-red.
  • Diamond conducts heat better than any substance - five times better than the next best element, silver.
  • Diamond has the highest melting point of any substance (3820 degrees Kelvin).
  • Diamond's atoms are packed closer together than the atoms of any other substance.

Diamond is only formed at high pressures. It is found in kimberlite, an ultrabasic volcanic rock formed very deep in the Earth's crust. The extreme pressures needed to form diamonds are only reached at depths greater than 150km.

diamond (in kimberlite)

Chemical composition - C
Hardness - 10
Specific gravity - 3.5
Transparency - Transparent to translucent in rough crystals
Colour - Variable, tends toward pale yellows, browns, greys, and also white, blue, black, reddish, greenish and colourless
Streak - White
Lustre - Adamantine to greasy
Cleavage/fracture - Perfect in 4 directions forming octahedrons / conchoidal
Crystal habit/mode of occurrence - Prismatic (isometric forms such as cubes and octahedrons)

Other specimens - Click the thumbnails to enlarge