“Carat” is the term used to refer to the weight of a diamond (not to be confused with the term “karat” referring the purity of gold). One carat is defined as one fifth of a gram or 200 milligrams, approximately the weight of a five pound note. A carat can further be divided into “points,” where one point is equal to 0.01 carat.
While the size of a polished diamond is related to its carat weight, it is important not to confuse carat weight with size, as there are other aspects of a diamond that can affect how large an individual stone actually appears—most importantly, the cut and shape of the stone. Proportions to which an individual diamond is cut, such as table and depth percentages must be taken into account, as shallower stones can appear larger than deeper stones of the same weight. Similarly, diamond shapes can have an impact, as elongated shapes tend to maximise carat weight, making them appear larger than rounded stones of the same weight.
Furthermore, the appearance of a diamond’s size can also be affected by how the stone is set in a piece of jewellery, as well the size of the wearer’s finger (in the case of rings), therefore it is important to take all of these aspects into consideration when choosing a stone.
History & Background
The term carat originates from the Greek and Arabic names for the carob tree - Keration in Greek and Qirrat in Arabic. The dried seeds of the Carob (or Locust) tree were once widely used by trading merchants as counterweights for weighing gold, diamonds, gemstones and pearls due to their relatively consistent weight and size. It is important to note however, that the term “carat” with reference diamonds is different to “karat” which is the value used for the purity of gold.
The Byzantine era used glass pebbles, based on carob seeds, for weighing coins, which weighed in at 196 mg, consistent with the average weight of an individual carob seed. However their use eventually diminished as it was discovered that despite their visual uniformity, the seeds were not actually consistent in weight. Many attempts were made to standardise the measurement of gemstone weight and it was only in 1907, at the Fourth General Conference on Weights and Measures that the “carat” was adopted as the official metric measurement for gemstone weights.
In 1913 the United States officially accepted the ‘carat’ as the gemstone measurement, and in 1914 the United Kingdom and Europe followed suit. By the 1930s, the majority of the diamond and gemstone industry had agreed to the standardised measurement, which is still in use today.
Carat Weight & Size
Dimensions play an important role in the appearance of a diamond. In addition to the carat weight, the distance across the top of the diamond must also be taken into consideration. A common misconception is that half a carat is half the size of one carat. In fact, a half carat is half the weight of one carat, but the millimetre difference on a round stone is only 1.35mm. The average measurement for a 0.50ct stone is 5.00mm, while the average 1.00ct stone measures at 6.35mm. Download PDF guide here.
While carat weight may indicate a diamond’s size, the shape and cut of a stone also play a large part in determining how large or small the stone appears. An elongated shape such as the Marquise cut may appear larger than a rounded shape such as the round brilliant even if the two stones share the same weight. To read about diamond shapes in more detail, please click here.
When comparing two stones of the same shape however, it is important to look at the cut grades and table and depth percentages, as shallower stones will tend to appear larger than deeper ones. Other aspects such as girdle width can also affect how large a stone appears, while not necessarily affecting the quality of the stone. To read about diamond cut in more detail, please click here.
Carat Weight & Price
Carat weight is one of the fundamental factors in determining the price of a diamond. As a general rule, the heavier the diamond, that is, the larger the carat weight, the more expensive it becomes. Price per carat is one of the best ways to compare the cost of similar diamonds. To calculate this, simply divide the cost of each stone by its carat weight. Because they are more scarce, larger diamonds are in much higher demand than smaller stones and therefore command much higher prices per carat. A diamond that is double the size of another can be up to four times the price. For example, a one carat diamond solitaire ring is nearly always more expensive than a diamond ring made up of smaller multiple diamonds whose total weight is one carat.
Undersize and oversized diamonds
Diamonds are usually cut to a rounded number in their carat weight, for example 0.70ct or 1.00 carat, rather than 0.69 or 0.99. Because of this, diamonds weighing just under the weight ‘barrier’ (e.g. 0.68ct or 0.69ct.), are scarce but highly recommended as they cost less than diamonds whose weight has been rounded up. Similarly, diamonds whose weight slightly exceed the barrier are referred to as “oversized” and also offer great value for money as they have not been rounded up.
Jewellery Settings & Finger Sizes
When choosing a diamond for a ring, it is also important to consider the length and width of the ring finger since a stone will appear larger (and therefore of heavier carat weight) on a longer and slimmer fingers. Comparatively, shorter and wider fingers tend to make stones seem smaller. Similarly, when deciding on a diamond for a particular jewellery setting, the appearance in size and weight can be altered depending on the type and proportions of the setting. Delicate and slender settings tend to enhance the size and prominence of diamond centrepiece while larger and more solid settings can make the stone appear smaller.
A note on vintage and antique jewellery
It is also worth mentioning how the standardisation of carat weight relates to antique, vintage and heirloom jewellery. If a piece was manufactured prior to 1913, the stated carat weight may not be accurate and the stone could be either smaller or larger when measured with modern values. Vintage jewellery is generally higher in value but anyone looking to purchase a vintage diamond piece should have it professionally appraised and measured.