The weight of diamonds and gemstones is measured in standardized metric carats (abbreviated ct) and must not be confused with the ‘caratage’ or purity of precious metals. The name ‘carat’ comes from carob seeds which were historically used as counterweights and are by nature unusually uniform.
One carat equals 0.2grams and 5 carats amount to 1 gram. 1 carat may further be subdivided into 100 points. Small diamonds below 1ct total weight are hence also known as ‘pointers’ in the diamond trade. Pointers of equal to or less than 15 points (pts) are referred to as melée or smalls. These are often measured in millimeters and commonly sieved for fast sorting.
Generally speaking the larger a diamond, the rarer it is, the higher its price will be. This correlation between weight (size) and price is essentially near perfect until a stone reaches a size of approximately 15 to 20ct. Beyond this range the per-carat-price relationship effectively becomes invalid and the price curve flattens. This is primarily due to the lack of practical use for giant stones as wearable ornaments.
The weight of a diamond has further indirect implications.
A well cut stone for example will usually show a sufficiently large face (surface area) due to an expected spread or diameter that is closely correlated with its true weight. Poorly cut stones on the other hand may be deep. However this relationship is not perfect and weight particularly difficult to estimate in smaller stones. When buying diamonds one is hence advised to compare the actual weight to the relative diameter in case of round stones or against the [length x width] figure when assessing fancy shapes. This is important as diamantaires will adjust pricing of stones accordingly.
Prices on a per-carat-basis of stones approaching the next size up, so called ‘oversizes’, are higher than those for stones situated at the bottom of the respective carat size class. For example a price premium will apply to a 1.80ct stone (all other factors equal) relative to the price of a 1.50ct diamond. Such pricing behaviour effectively smoothes out the price curve.
This however does not contradict the percs of buying a ‘shy’ carat diamond. These are stones just below a psychological ‘size’. A good example would be a 1.98ct instead of 2.00ct diamond. Since prices jump at certain weight boundaries, significant savings would be expected. Such a ‘shy’ two carat stone would essentially still look the same size as a true two carat stone and be considered a savvy buy.
Lastly certain cultural superstitions affect marketing of particular sizes nowadays. 8ct diamonds for example attract prosperity according to Chinese who are thus willing to pay premiums for such stones. Contrarily 4ct diamonds are avoided by the Sino market as the number is a harbinger of bad luck.