The origin of diamonds

Diamonds, with their fire and brilliance, are remarkably beautiful. Perhaps even more remarkable is the fact that they exist at all! The diamond in your ring or piece of jewellery has survived incredible odds to be there.

Diamond formation

Diamonds were formed over 990 million years ago and approximately 150 kilometres below the earth’s surface. At such a depth, the pressure is almost fifty thousand times that on the earth’s surface. Even more dramatically, diamonds are formed at temperatures between 900°C and 1300°C.

Over time, this intense heat and pressure builds up, forcing the diamonds and rocks up toward the earth’s surface in a volcano-like explosion. This blast creates a deep, wide hole known as a pipe into which most of the diamonds settle. This is known as a primary deposit and the diamonds must be mined from the pipe deep in the earth.

Secondary or alluvial deposits are diamonds that have travelled up to the earth’s surface and then been washed or eroded away. These diamonds often settle into the waters of nearby rivers.

Diamond composition

Diamonds are made up of pure carbon atoms that are all equidistant from one another. Diamonds belong to the cubic or ‘isometric’ crystal system where the atoms are linked in a regular three-dimensional lattice with a repeating crystalline pattern.

Interestingly, the nature of a diamond’s crystal structure gives it incredible hardness and also enormous compressive strength. A diamond will withstand a force of 1.25 million tons per square inch.

Diamond properties

Hardness
Diamonds are generally regarded as the hardest substance known to man and are extremely resistant to heat and chemicals. This hardness is a great advantage. Gemstones are resistant to scratching and abrasion, which means that a finished diamond will retain its brilliance and polish. No other gemstone can match a diamond’s standard of polish. Diamonds are also used in industrial tools for cutting, drilling etc.

OPTICAL PROPERTIES

Refractive index
The refractive index is a measure of how well a substance can refract light. A diamond’s refractive index is very high and, interestingly, does not vary much from one diamond to another.

Brilliance
The brilliance or ‘life’ of a polished diamond is the amount of light that is reflected back to the viewer. A diamond’s brilliance can be increased by its cut [link]. Facets that are polished at the correct angles will provide maximum brilliance.

Lustre
Lustre is the surface gloss on a polished diamond. A diamond’s lustre mainly depends on its refractive index and the quality of polish.

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’.