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Properties & Myths

Diamond (greek adámas, „invincible, unbreakable“) is the hardest natural material. It is the cubic modification of the element carbon and a naturally occurring mineral. Mohs scale of mineral hardness gives it the highest ranking of 10 (1 being the softest). Hardness is defined as resistance to scratching. An interesting comparison: Corundum (saphire and ruby) get ranked 9 at the Mohs scale, however, diamond is still 140 times harder! The reason for this is the strong covalent bonding and the three dimensional crystal stucture diamonds.

Different atomic struture

Diamond

 


- 1 carbon atom binds 4 more atoms
- Three dimensional structure
- Only covalent (strong/energy-rich) bonding

Graphite

  


- 1 carbon atom binds 3 more atoms
- Layer structure
- Covalent bonding only in the horizontal layers

In the crystal structure carbon atoms are arranged tetraedically. That is, every atom has 4 symmetrically adjusted bondings to its neighbours.

Comparing: The graphite in a pencil is the same material as diamond chemically speaking. However, the carbon has a hexagonal layer structure. The strong covalent bondings are responsible for a diamonds hardness. Graphite has the same covalent bondings - in the horizontal layer. Die layers itself though are held together by the weak Va der Waals bondings on the vertical axis. That is the secret behind this mystery.

So how can you work on a diamond at all if there is no harder material? Right, using another diamond. It´s hardness depends on the direction of the crystal growth. Depending on the various form of the crystallization (octahedron, tetrahedron, dodecahedron or cube) there different degrees of hardness within the crystal. This dependency on crystal growth direction is called anisotropy. It is also the reason for a diamond´s excellent cleavability- according to the cleaving direction of the octahedron planes, the cleaving planes. Cleavage is defined as the tendency of minerals and crystals to break at certain parallel planes of the crystal structure. This feature came in handy early on in the diamond manufacturing process.


Physical properties

Hardness10 (Mohs scale)
CleavagePerfect (according to cleavage planes)
FractureConchoidal (shell-like)
Density3.52 g/cm³
Refractive index
2.417
Dispersion
0.025 (CF) und 0.044 (BG)
Thermal conductivityAbout 6 times higher than copper and 5 times better than silver
Electric conductivityType Ia, Ib and IIa diamonds do not conduct electricity. Diamonds of type IIb are semiconductors
CrystallizationCubic, mainly octahedronBut also hexahedron (form of dice) and tetrahedron
Oxidation
Above about 720° C in oxygen
Myths


Diamonds are unbreakable
Diamonds may be imperishable, but not unbreakable. The hardest substance on earth may be immune to acids but due to its physical properties mentioned above it has a few weak spots as well: Cleavage and fracture make diamonds susceptible to blows and percussions. That means, the door of a car can damage the diamond when hit against it by accident. Also fire: When working on your ring with the blow torch the gold smith needs to be careful not to batter the precious stone with burn marks. Diamond oxidates above about 720° C in oxygen.

Diamond are rare

Not really: Every year about 130 million carats of rough diamonds are produced. However, only between 25-50% of these are suitable for jewelry. The rest is used as industrial diamonds. Of the remaining there is only a small percentage of high quality diamonds- especially with increasing carat weight. The limited supply of high quality diamonds really drives the price here. In addition to the natural diamond production more than 4.5 billion carats of synthetic diamonds are produced- mainly in China. The material diamond is not rare by itself. Diamonds of high quality are rare and here the famous saying is valid once again: Small details make all the difference.

Diamond is the most expensive precious stone

Diamonds in jewelry are desired, rare and expensive. However, depending on size and quality there are in fact precious stones that yield higher prices per carat than diamond: These can be extremely rare rubies, saphires, emeralds or alexandrites.