In 1953 the GIA (Gemological Institute of America) released the International Diamond Grading System which became the industry standard. Based on the 4 C's (Cut, Clarity, Colour, Carat Weight), the system is still used today.
Arguably the most important factor in grading a diamond. The cut gives the diamond its' fire and brilliance. The cut is made up of three factors: proportions, symmetry, and polish, and it remains the most difficult 'C' to grade. The proportions relate to the size of the table (top individual facet), the depth of the pavilion (bottom part of the diamond), and the angle of the crown (top part of the diamond). A poorly cut diamond allows the light to escape through the bottom and the sides of the stone, which makes the stone look dull/flat, and reduces the fire and sparkle that makes the diamond such a special gemstone.
The clarity of a diamond refers to the presence (or lack) of inclusions either internally or externally. These inclusions are formed under severe heat and pressure underneath the surface of the earth. These inclusions can also be used to identify a diamond. Whilst the vast majority of the worlds' diamonds have some form of inclusion, there are 'flawless' stones which command very high prices due to the exceptionally rare nature. The GIA system for grading the clarity of a diamond has 11 grades which considers the size, nature, position, colour or relief, and quantity of clarity characteristics visible under 10× magnification:
- Flawless (FL) - No inclusions or blemishes are visible to a skilled grader using 10× magnification
- Internally Flawless (IF) - No inclusions and only blemishes are visible to a skilled grader using 10× magnification
- Very, Very Slightly Included (VVS1 and VVS2) - Inclusions are difficult for a skilled grader to see under 10× magnification
- Very Slightly Included (VS1 and VS2) - Inclusions are minor and range from difficult to somewhat easy for a skilled grader to see under 10x magnification
- Slightly Included (SI1 and SI2) - Inclusions are noticeable to a skilled grader under 10x magnification
- Included (I1, I2, and I3) - Inclusions are obvious under 10× magnification and may affect transparency and brilliance
Most commercially traded diamonds are graded between VS1 and SI2.
The colour of a diamond is the most visible aspect of the 4 C's, and is also the easiest way for a non-specialist to begin judging a stone. Many colour distinctions are so subtle that they can only be correctly identified by a trained eye. As with the International Diamond Grading System, the GIA colour-grading scale has been adopted as the international industry standard. The system gauges how close to colourless a diamond is ranging from D (colourless) to Z (light yellow/brown). There is a clearly defined spread for each colour grade. The colour of a diamond can only be truly assessed when compared to a master stone that has a known colour, which must be also be done under controlled light and surrounding conditions.
The D-Z scale does not account for coloured diamonds such as red, pink, blue, and yellow, although a separate scale does exist. This scale ranges from Vivid to Light.
The weight of a diamond is measured in Carats, although this can often be misinterpreted as the size of the stone. Equal to 0.2g, one carat consists of one hundred points, and will always be rounded to the nearest hundredth of a carat.
It is very important to note that the value of a diamond is based on a combination of all of the 4 C's.