Diamond grading covers numerous aspects of each individual diamond, but there are four criteria which are critical to understand: cut, clarity, colour, and carat weight. There are additional ratings and measurements noted on a diamond certificate, but they generally fall under the diamond’s grade in one of the Four Cs. For example, measurements listed such as “depth” and “table” are part of the description of a diamond’s cut.
Diamond cut is perhaps the most important of the four Cs, so it is important to understand how this quality affects the properties and values of a diamond. A good cut gives a diamond its brilliance, which is that brightness which appears to come from within the stone. The angles and finish of any diamond are what determine its ability to reflect light, which leads to brilliance. As shown in the images below, when a diamond is well-cut, light enters through the table and travels to the pavilion where it reflects from one side to the other before reflecting back out of the diamond through the table and to the observer’s eye. This light is the brilliance we mentioned, and it’s this flashing, fiery effect that makes diamonds so mesmerizing.
Diamond cut examples
In a poorly cut diamond, the light that enters through the table reaches the facets and then ‘leaks’ out from the sides or bottom of the diamond rather than reflecting back to the eye. Less light reflected back to the eye means less brilliance.
Diamonds are graded for clarity under 10x loupe magnification. The two international clarity grading systems are GIA and CIBJO, whilst the terminology is a little different the meaning is the same.
|Loupe clean||F – IF||Flawless or Internally Flawless (two grades). No internal inclusions. Very rare.|
|VVS1 – VVS2||VVS1 – VVS2||Very Very Slightly Included (two grades). Minute inclusions invisible to the naked eye and seen only with difficulty under 10x magnification.|
|VS1 – VS2||VS1 – VS2||Very Slightly Included (two grades). Minute inclusions invisible to the naked eye but visible under 10x magnification.|
|SI1 – SI2||SI1 – SI2||Slightly Included (two grades). Minute inclusions seen with difficulty by the naked eye.|
|P1 – P3||I1 – I3||Included (three grades). Inclusions visible under 10x magnification as well as to the human eye.|
When jewellers speak of a diamond’s colour, they are usually referring to the presence or absence of colour in white diamonds. Colour is a result of the composition of the diamond, and it never changes over time. Because a colourless diamond, like a clear window, allows more light to pass through it than a coloured diamond, colourless diamonds emit more sparkle and fire. The formation process of a diamond ensures that only a few, rare diamonds are truly colourless. Thus the whiter a diamond’s colour, the greater its value. There are a number of systems used to describe colour, the GIA use an alphabetical system starting a “D” (colourless) and ranging to “Z”, whilst CIBJO use descriptive terms from “Colourless” ranging to “Yellow”.
Diamond Colour Scale
NOTE: Natural fancy coloured diamonds do not follow this rule. These diamonds, which are very rare and very expensive, can be any colour including pink, green, blue and yellow.
A carat is a unit of measurement, it’s the unit used to weigh a diamond. One carat is equal to 200 milligrams, or 0.2 grams. The word “carat” is taken from the carob seeds that people once used in ancient times to balance scales. So uniform in shape and weight are these little seeds that even today’s sophisticated instruments cannot detect more than three one-thousandths of a difference between them. The process that forms a diamond happens only in very rare circumstances, and typically the natural materials required are found only in small amounts. That means that larger diamonds are uncovered less often than smaller ones. Thus, large diamonds are rare and have a greater value per carat. For that reason, the price of a diamond rises exponentially to its size.