Diamond Quality Guide - Rock C Jewellers | Watches, Jewellery & Engagment Rings

Diamond Quality Guide

Diamond is a colourless exceptionally hard mineral made up of pure carbon. Diamond is formed at great depth in the earth (between 150-300km below the surface) at great temperatures and pressures. By a type of volcanic eruption, diamond is pushed up through volcanic pipes of diamond bearing rock, this rock is known as kimberlite.

Diamonds are mined in many countries, the top five producers being: Australia, Russia, South Africa, Zaire and Botswana. These are called fancy diamonds. The colouring of the diamond is produced by certain elements which mix with the carbon to create the colour. An example of this is a blue diamond which is coloured by the element Boron. Naturally coloured diamonds can be more expensive than colourless diamonds due partly to rarity.


When purchasing diamonds, equal consideration should be given to colour, clarity and cut, so that it is not the carat weight alone that defines the value of your diamond.

Large diamonds are much rarer than smaller stones, and therefore larger gems are extremely expensive. For example, a 1-carat stone would be worth more than twice as much as a ½-carat stone of similar colour, clarity and cut.

Diamonds are weighed in carats. A carat is equal to 0.2 grams, and is divided into 100 points, therefore a 25 point diamond = ¼ or 0.25 carat. The term carat is derived from carob seeds that were originally used to measure the weight of diamonds. These seeds, from the Ceratona Siliquia tree, are a remarkably uniform weight of 0.2 grams, and were used to measure the weight of diamonds until the system was standardised at the start of the 20th century.

The cut of adiamond is the only characteristic which is not natural, but is directly influenced by man. The cut is not just the shape and style of the polished stone it also includes the proportions, symmetry and quality of polish of the finished stone.

It is the cut of the diamond that gives it its sparkle, known as brilliance, and the rainbow of colours known as fire. An equally proportioned, symmetrically cut diamond bounces light around internally from facet to facet, acting like a prism and a mirror, before returning the light back through the top of the diamond.

To achieve this maximum return of light, the pavilion depth, crown angle and table size must all be cut within a range of ideal proportions. If these proportions are not correct, total internal reflection does not occur, and light leaks from the pavilion facets reducing fire and brilliance.

Diamonds are cut in many fancy shapes, but the most popular is the round brilliant cut, with its eightfold symmetry. This cut also makes best use of rough octahedral diamond crystals, with the minimum of waste.

Diamonds in the normal colour range are graded by their lack of colour; therefore a stone with the highest colour grades has little or no visible colouration. Colour grades can cause dramatic variations in price, although distinguishing between the top colour grades in smaller mounted stones is almost impossible to the untrained eye.

The colour of diamonds is designated using the following grading scale:

  • •D – a totally colourless diamond, the highest grading possible, and as such, rare and valuable.
  • •E – colourless, with only a trained and experienced diamond grader able to detect the minute hint of colour.
  • •F – still a colourless stone, but with a minor hint of colour, again only detectable by a trained grader.
  • •G – almost colourless, although a trace of colour may be seen when compared with higher grades. Still a very high quality grading.
  • •H – almost colourless, but a hint of colour may be discernible.
  • •I – nearly colourless, although a trace of colour will be detected, with difficulty.
  • •J-Z – tinted stones, outside of our standard range.
  • Each grade represents a spread or range of colour, not just one point along the series. In this range, a D colour diamond will always be the most valuable if all the other factors of clarity, carat and cut are equal.

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