Instead of grams or kilos, diamonds are weighed in carats (not to be confused with gold’s Karat which signifies purity). This simply denotes a measuring scale where each 1 carat = 0.2 grams (0.50 carat = 0.1 gram and 5 carats = 1 gram).
As the carat weight increases, so does the size of a diamond. However, confusingly, this relationship is not a linear but rather a curve - so a 2.0ct diamond will not appear twice as big as a 1.0ct diamond as shown below.
Expert Tip 1
When comparing similar diamonds try to look at the measurements. Some diamonds with similar weights can vary significantly in measurements. Pick the one with wider measurements, and you will gaina larger diamond surface area for the same value.
Expert Tip 2
Certain diamond sizes are highly sought after (e.g. 0.50ct, 0.75ct, 1.0ct) and prices can vary dramatically depending on the carat weight. A 0.90ct diamond can be 10-20% better value than a 1.0ct diamond of the same quality but can appear almost identical in size but very different in price.
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.
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.