Composition and properties of diamonds

All the atoms are equidistant from each other.

Diamond belongs to the cubic, or ‘isometric’ crystal system. The most commonly seen crystal structures or arrangements, known as ‘morphologies', are:

• Octahedron (8 faces)
• Cube (6 faces)
• Dodecahedron (12 faces)

Hardness
Diamond is generally recognised as the hardest substance known to man. Contrary to some reports, cubic boron nitride is not harder than diamond, although it is a very useful abrasive and performs better than diamond in some industrial applications because of its chemical properties.

A diamond’s crystal structure gives it not only its hardness but also its enormous compressive strength. It will stand a force of 1.25 million tons per square inch, which is more than three times the weight of the Empire State Building.

The Mohs’ scale was developed in 1822 by Friedrich Mohs, and has been used to assess hardness for over 150 years. There are ten minerals in this series. Diamond, as the hardest mineral, is top of the scale.

However, this is not a linear scale but merely an order.  Every mineral in the scale will scratch all minerals below it on the scale. There is very little difference in hardness between talc and gypsum at the bottom of the scale, but an enormous difference between corundum and diamond at the top of the scale.

The hardness of diamond is an important property. As an industrial tool it has many uses and modern industry is highly dependent upon it.  As a gemstone, it is resistant to scratching and abrasion, which ensures that a finished gem will retain its brilliance and polish.

In addition, because of its hardness and the unique way in which it is manufactured, diamond polishes very slowly. It forms an unusually flat, finely-polished ‘adamantine’ surface, with very sharp, straight edges between facets. No other gemstone can quite match this standard of polish.

Thermal conductivity
Thermal conductivity is the ability of a substance to pass heat from one area to another. Diamond has the highest coefficient of thermal conductivity of any known substance because the closely-packed crystal structure conducts heat very quickly.

The thermal conductivity of diamond is five times higher than that of copper. This explains why a diamond feels cold to the touch when first picked up but quickly becomes warm from the heat of your fingers.

Specific gravity
The specific gravity of diamond is 3.52 (i.e. its density is 3.52g/cm3). This means that diamond is a dense and relatively heavy material, more than three-and-a-half times heavier than the equivalent volume of water at 4°C. In diamond mines, this property is used to help separate rough diamonds from the accompanying rock.

Refractive index
‘Refractive index’ (‘R.I.’), is a measure of how well a substance can refract light. The R.I. of natural diamond is very high, at 2.4175. In addition, the R.I. does not vary very much from one diamond to another. The normal range of refractive indices for glass is between 1.50 and 1.70.

Life/brilliance
The ‘life’ of a polished diamond is regarded as the amount of light that is reflected back to the viewer. The term ‘life’ is also referred to as ‘brilliance’. If the stone is cut to what are widely recognised as good proportions, for example if the facets have been polished at the correct angles, then brilliance will be increased.

Lustre
This is the surface gloss on a polished diamond, which to a large extent depends on the refractive index and quality of polish. Gemmologists describe diamond’s brilliant lustre as adamantine: ‘diamond-like’.

Very few gemstones have this type of lustre. Most well-known gemstones have a lustre that is described in one of the following ways: ‘sub-adamantine’, ‘vitreous’, or glassy, ‘metallic’, ‘pearly’, ‘waxy’, etc.

Fire
This is the play of colours that can be seen from the crown of a polished diamond. As light enters the diamond it is refracted and broken up into the colours of the spectrum and reflected back. The resulting rainbow-like colour flashes are called ‘fire’.