The most misunderstood of the 4Cs that refer to diamonds and other valuable gemstones is that of a stone’s weight compared to a stone’s size. Carat weight is not the same as size or volume. When considering carat and cut together in one stone, a large carat diamond that has not been cut well may appear smaller than a smaller carat diamond that has been cut well. Different gemstones have different densities so two different stones’ same carat weight will not necessarily mean they are of the same size.
The first diamond traders used small, uniform carob seeds from the Carob or Locust Tree on their balance scales as counterweights, from which the name carat (abbreviated as ct) evolved; however, the seeds were not consistent in weight and a more accurate metric weighing measure was found. In 1907 at the Fourth General Conference on Weights and Measures, the carat became the official metric measurement for weighing gemstones. This was adopted by the United States in 1913 and by the United Kingdom and Europe in 1914. By the 1930s most of the world’s gemstone industry adopted this measure which still exists today.
Worldwide a carat weight is the measurement of how much a diamond weighs, one carat being equal to 0.2 grams (0.007 ounces) or 200 milligrams. Each carat is subdivided into 100 points, enabling precise measurements to be made. Do not confuse carat (ct) with karat (K), the latter referring to the purity of gold as in eg 9K or 18K gold.
Each diamond sale should come with a grading report which shows the exact weight of the stone, eg ¾ of a carat is 0.75ct. In the diamond industry the price of a stone is referred to by price per carat or P/C and not the overall cost of the stone. When purchasing a diamond the exact carat weight is of paramount importance in the pricing of the stone.
Diamond Carat Weight Measurement
Very accurate electronic micro-balances are used to weigh diamonds to the exact fifth decimal place (the nearest ten-thousandth of a carat). To determine the stone’s proportions, measurements and facet angles, an optical measuring device is used.
A loose diamond can be weighed on an electronic scale for an accurate weight assessment. However, for mounted diamonds which cannot be removed for weighing, another process is used which gives an estimated approximate weight using various formulae instead of a scale. The key measurements of the stone are necessary in order to use these formulae.
An example for a round or oval diamond’s weight is the following formula – diameter in mm x diameter in mm x depth in mm x coefficient 0.0060.
The diameter is the straight line through the centre of the stone that connects two points on the stone’s girdle or outer edge. The depth (total height) is the vertical distance from the top facet or table to the bottom of the stone.
An example of a round stone with a diameter of 5mm and depth of 3mm is 5 x 5 x 3 x 0.0060 = 0.05ct
For square and rectangular cuts the formula changes to length x width x depth x coefficient. The latter depends on the length to width ratio as follows:
L/W ratio Coefficient
In order to select a perfect stone it is vital for the purchaser to understand precisely the factors that are necessary to understand what carat weight entails.
The cost of a stone increases significantly per carat weight, but cost also depends on the grade of cut. In order to save money, it is wise to ‘buy shy’.
This means that a carat weight slightly below either the full or half carat marks will give you good value. An example would be selecting a carat weight of 1.4 instead of 1.5 as the size will be barely noticeable and almost impossible to detect because the weight is distributed over the entire stone, but the price will probably be considerably less. However, it may be difficult to find stones that are in the median weight ranges – ie neither a full nor a half carat, as the cutters try not to drop to a lower carat value. The weight of a diamond below one carat may be called a pointer eg 0.25 carats is known as a twenty five pointer. A 1.09 carat stone would be known as a one point zero nine carats.
‘Magic sizes’ are particularly half carat, three-quarters carat and one carat. Sizes below the ‘magic sizes’ lower the P/C value. Prices increase as weight and size increase as larger diamonds are rarer and the supply is limited. However, other 4C factors such as clarity and colour also come into play, together with cut and carat weight. A diamond’s value is dependent on all 4Cs, not just one of the Cs.
Fancy coloured stones, and especially the larger ones, are rare finds and thus significantly more valuable. Less than one in a million rough stones yield a finished one carat diamond as most rough diamonds are either too small or too inferior in quality to produce quality stones for the jewellery market.
If the budget for a diamond sale is limited, and carat weight is the most important factor, then the points to consider in order for the best value are a good cut, Slightly Included (Sl1-Sl3) clarity and a colour grade of Near Colourless (G-J).
The size of a diamond increases with carat weight but this is a curve and not a linear relationship – see below where a 2.0ct stone does not appear to be twice the size of a 1.0ct stone.
The most popular shape is the round brilliant cut diamond which has 57 facets. This cut optimises both brilliance and dispersion and is found in more than 60% of world diamond sales.
Factors to consider when purchasing a Diamond
A diamond’s diameter is the distance in millimetres across the top of the stone as seen when viewed from above when set into a ring.
The view from above a stone, the table or crown area, is the total surface area and is what is seen at first glance, often the most important aspect of a diamond’s appearance.
Look at a diamond’s measurements when comparing similar weight stones. A stone with wider measurements will have a larger surface area for the same value. The shape will also affect the size – a longer shape such as a marquis diamond is visually larger than a round or square stone. An oval shape may appear larger than a round shape if their surface areas are the same, but this is simply a perception, not a reality, resulting in the illusion of a larger size per carat weight. Every diamond is unique – there will never be two identical stones.
Cutting grades of Very Good, Ideal or higher will make a stone seem larger due to the amount of light reflected out of the top. Conversely, a higher carat weight may look smaller with lower cut grades such as Good, Fair or Poor.
Another consideration is that a diamond will appear larger on a size 4 finger than on a size 8 finger and the type of diamond shape will affect the setting.