The weight of a diamond is measured in carats - one carat is 0.2 grams. This is the easiest factor to determine. However, weight is related to but is NOT the same as size! Two diamonds of the same shape cut from the same rough (with equal carat weight, colour and clarity) can be different size depending on how well they have been proportioned. The better cut diamond will appear bigger than a less well-cut diamond! Also as diamond prices go up exponentially with carat weight, doubling the carat weight from say 1 carat to 2 carats more than doubles the price, but the visible size of the table (the largest surface at the top of the diamond) goes up by only ca. 30%. As about 50% of the rough diamond is lost when cutting, the larger the diamond the rarer it is in nature hence the price increases most steeply with carat. Approximately 250 tons of ore must be mined and processed in order to produce a single, one-carat, polished, gem-quality diamond.
Overall, for any given carat – the price also varies considerably with colour and clarity. For example for a loose 1 carat round brilliant diamond the trade price for a D (the best blue white colour) Internally Flawless (loupe-clean clarity) is more than FOUR times the price of a 1 carat H (white colour) VS2 (eyeclean++ clarity) diamond! This difference reflects rarity rather than visible differences: less than 0.003% of all 1 carat diamonds have the combination D colour and IF clarity! Only 8% of the jewellery quality diamonds even have the combination of G VS2 or better!
