Colour: The value of a diamond increases as it becomes more and more colourless. A completely colourless diamond (also called a blue-white or colour D) tends to have the highest value.
The colour scale ranges from D (the most colourless) to Z (the diamond has traces of colour).
The exception to this is the fancy colour diamonds, where these diamonds can be any colour. The most valuable colours are blue, red, and pink diamonds.
Clarity: The clarity of a diamond refers to how many imperfections reside in the stone (internally or externally). These imperfections are called inclusions, and appear as black or white specks or lines in the stone. The less inclusions in the stone (or the smaller the inclusions), the higher the clarity of the diamond, and consequently the more valuable it is. Furthermore, the value of the stone also depends on the location of the inclusions, where inclusions located on the edges of the diamond are better than those located in the centre, for example.
All clarity is graded at 10x magnification, and the clarity of a diamond can range from FL (flawless, where there are no inclusions at 10x magnification) to VS1/2 (very very slightly included, where inclusions are barely visible at 10x magnification) to I3 (where there are very obvious eye visible inclusions without magnification).
Billions of years ago when your diamond was formed under intense pressure and heat, irregularities may have formed within its structures. For example, fissures may have developed, or a crystal may have been captured within the structure of the diamond (where there have even been cases of diamonds with coloured gemstones, such as garnets, inside).
Cut: The cut of a diamond is often considered to be the most important of the four Cs. This is because if the diamond is not cut to the correct proportions then it will not sparkle.
The choice of cut is determined by the shape of the rough diamond, the location of inclusions, and the maximisation of the weight (carat) of the stone.
The most popular cut is the round brilliant, as this has the highest refraction of light, which means that it is the sparkliest cut. The round brilliant cut has been perfected by mathematical and empirical analysis.
Carat: The term carat refers to the weight of a diamond, where 1 carat is equal to 0.2 grams. The bigger the carat of the diamond (depending on the cut, colour and clarity), the more valuable the diamond.
In the trade, we say that there are 100 points to a carat to allow for very precise measurements of weight. These are often referred to as “pointers”, for example a 50-pointer is equal to half a carat (0.50 carats).