Diamond size can be estimated by its weight expressed in carats. A 1 carat diamond weighs 0.2 grams and has a diameter of 6.3-6.5 mm.
History of Carat
The “Carat” name comes from the carob tree. In the past, people used to weigh gemstones against carob seeds, as they believed all carob seeds were almost the same weight (0.197 grams). In 1871, the measuring system was standardised by the Syndical Chamber of Diamond Merchants in Paris, who proposed an ‘international carat’ – a measure equivalent to 205 milligrams. A carat of 200 milligrams – exactly one-fifth of a gram – was much more convenient, and it was finally unanimously accepted by the General Conference of the Metric Convention in Paris in 1907.
Carat as Weight
What is a forty pointer? One carat is divided into 100 points. The weight of stones above one carat is normally expressed in carats, but for stones under one carat the weight is normally expressed in points. A stone, that weighs 0.40 carats is a “forty pointer” as it weighs forty points. If a stone weighs 1.72 carat, one says “one carat seventy-six”.
The weight of diamond is indicated to two decimal points. For example, 1.34 ct. This gives an accuracy of 0.002 grams. The third decimal point is rounded down as long as it is smaller than 9. When the third decimal point is 9 then it is rounded up. For example, 0.298 ct = 0.29 ct, 0.299 ct = 0.3 ct.
The influence of carat on the price
If we compare stones of the same quality (colour, clarity, cut), the general rule is that the bigger the stone the more it is worth. Interestingly, diamond prices do not grow in a linear way as they grow in size. A stone of 1 ct costs more than two stones of 0.5 ct of the same quality. A one carat diamond costs approximately 3-4 times more than a half carat stone. A large stone is rarer to find and thus it has greater value than several stones of an equal total weight.
The price of diamond does not follow a simple linear growth pattern. There are a few so-called “magic boundaries”, around which prices ‘jump’ drastically. From the moment the weight of a diamond goes above the magic boundary, its price also shoots up. These borderlines include 0.30 ct, 0.40 ct, 0.50 ct, 0.70 ct, 0.90, 1.00 ct, 1.50 ct, 2 ct, 3 ct, 4 ct, 5 ct and 10 ct. For example, a stone just above one carat can be about 30% more expensive than a stone just below. As the difference in size is tiny, sometimes invisible at the first glance, an engagement ring with a 0.49 ct diamond will look the same as 0.50 ct but it will be significantly cheaper. However, stones just below the magic borderline are not easy to find, as polishers aim to hit the higher price line to have more value for the diamond.
The content of gold is indicated by karats or thousands. The same word (but spelled differently) is used to indicate gemstone weight (carat, ct) and gold purity (karat, kt). The gold karat expresses pure gold/other metals ratio. Pure gold is 24 karat or 1000/1000. 18 karat gold, 750/1000, has ¾ of pure gold and ¼ of other metals.