The quality and value of diamonds are measured by four characteristics known as the 4C's. The 4C's relate to a diamond's cut, colour, clarity and carat weight. The quality of a diamond is measured by its cut, colour and clarity. The carat weight measures the size of the diamond. Of all the 4C's, cut is the characteristic directly influenced by man; colour, clarity and carat weight are all dictated by nature.

Cut

A diamond in its natural, uncut state is described as a "rough diamond". Its natural appearance so resembles a glass pebble that most people would pass it by without a second glance. It is the skill of the diamond cutter that unlocks the brilliance for which diamonds are renowned.

If two identical diamonds are placed side by side and one is less brilliant and fiery than the other, the fault lies in the cutting. Such a stone cannot demand as high a price as a well-cut diamond.

It is important to distinguish between cut and shape. Some of the more popular shapes of diamonds include Round Brilliant, Oval, Marquise, Pear, Heart and Emerald. Within each of these shapes, however, it is the cut that determines the quality of the stone. For example, most diamonds are cut with 58 facets, regardless of their shape.

Colour

A diamond's colour is one of the most important factors in determining its value. The nearer a white diamond is to being absolutely colourless, the more rare and valuable it is. The graduations in colour are so subtle that intricate international grading scales have been devised.

Diamonds are graded into categories defined by letters. The colour range from exceptional whites (categories D, E and F) to tinted colours (categories M to Z). The best way to pinpoint a diamond's true colour is to place it next to another diamond that has previously been graded.

There are also fancy coloured diamonds and these are graded according to their intensity of colour, not lack of it. There are a variety of reasons for diamonds to be coloured. The most common causes, or suggested causes, for the colours yellow, green, blue, brown and pink are described below.

Yellow:

When nitrogen combines with the diamond crystals during the formation stage it causes a surplus electron in the bonding. This surplus electron absorbs blue light, thus giving off a yellow colour. Yellow diamonds also occur when aggregates of three nitrogens combine and cause surplus bond.

Blue:

The elements of boron may also be substituted within a diamond during its formation. Boron absorbs red light, hence in the absence of nitrogen, diamonds containing boron are blue in colour. An example of a diamond containing boron is the famous Blue Hope diamond. Diamonds containing boron also exhibit unusual electrical properties and are semi-conductive in nature. Hydrogen is another impurity that, in high quantities, can cause grey or blue colouring in diamonds. However, these diamonds are not semi-conducting.

Green:

A vacancy in the regular lattice of atoms within a diamond results in a green colouring. Carbon atoms being knocked out of their regular position by other particles cause vacancies. The depth of colour usually extends about 2mm below the diamond's surface. At extremely high temperatures the vacancies can become mobile and can combine with nitrogen to form other colours such as mauve, orange, blue or gold.

Brown:

It has been suggested that dislocations in the regular lattice of atoms, caused by severe forces deep in the earth, may be responsible for the brown colouring of champagne and cognac diamonds. The dislocated bonds may affect the light wavelength, thus producing a diamond which is coloured, but which contains no impurities.

Pink:

It has also been suggested that combinations of dislocations, vacancies, and non-nitrogen impurities cause the much sought-after colouration in pink diamonds. However these theories are still being developed.

Clarity

Inclusions may not always be visible to the naked eye, however they do interfere with the passage of light through the diamond. Therefore the fewer inclusions a diamond has, the more valuable it is.

Like colour, clarity is also categorised using international grading scales. The categories of clarity are based upon the number, size and position of the inclusions within the diamond. Gradings range from flawless, and internally flawless, through very small and small inclusions, to imperfect. The clarity gradings are described as follows:

Carat

A carat is the unit of measure used to determine the weight of a diamond. The term "carat" is derived from the original method of using carob tree seeds to weigh diamonds. One seed from this tree was equivalent to one carat.

The actual weight of one carat is now established at 0.2 grams. To assist in accurately describing the weight of diamonds each carat is divided into 100 points. Diamonds of less than one carat in weight are known as "pointers". For example, a 0.15 carat diamond would be called a "15 pointer".

Diamonds are usually weighed prior to setting for more accurate measurements. Diamonds are priced per carat, according to their size and quality. Although the carat weight of a diamond is indicative of its size, it is not necessarily indicative of a diamond's quality. Therefore, where two diamonds have the same carat weight, the one of better quality will command a higher price per carat.