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Simply put carat refers to the weight of the diamond. Carat is a metric system of weight and 1 carat equals to 0.20 grams. Consequently 5 carats is 1gram. Carat is abbreviated as “Ct.” and the diamond weight is expressed up to 100th of a carat for example 0.75ct, 1.20ct, 2.12ct and so on. The unit carat is also used to express weight of other gemstones as well.
Consider a diamond that weighs 0.98ct, the different ways of expressing its weight could be:
A customer should always know the exact weight of the diamonds especially the center stone. That makes it possible for the customer to perceive a value of a diamond more accurately.
The carat system came from a carob seed which was used to weigh the precious stones before the 20th century as precise scales didn’t exist at the time. Carob seed comes from the locust tree, the seeds grow in pods that are used for flavoring and for livestock feed. Because the small seeds are fairly uniform in size and weight, they provided a fairly consistent basis for gem weight. Early gem merchants and jewelers used them as counterweights in hand-hald balance scale. Consequently that carat weight wasn’t always consistent and could be anywhere between 0.95 to 1.07 metric carats, until the carat was standardized to be 0.20 gram in the the early 20th century. (Source: GIA Text Book – Diamond Essentials)
The cost of diamond almost always is computed using it’s weight. A per-carat value is determined and that per carat value is applied to the weight of the diamond to arrive at a per stone price.
Diamond price = Per carat price x carat weight
Diamond price = $4500 x 0.75ct = $3375
While it’s true that diamond’s weight and it’s apparent size are positively correlated, it’s not entirely always the case. While a 0.50ct round diamond will most certainly appear bigger than 0.10ct diamond the same is entirely not always true when comparing diamonds of near weights say for example comparing 0.40ct diamond to a 0.50ct diamond. It is entirely possible for a 0.40ct diamond to appear bigger than 0.50ct diamond. So where’s the weight going? The answer is CUT. When a diamond is poorly cut it may appear smaller than it’s size. When buying diamond a customer should always consider the cut first thing so they don’t end up with a one carat diamond that appears like a 80-points.
We learned in the previous section that diamond is priced according to it’s weight and that there’s price-per-carat. But what influences that price-per-carat? It’s the weight of a diamond. Consider a half carat F-VS2 diamond: it’s price per carat is for example $3000 and the price of diamond consequently would be a 1500$ and two half carat (total weight 1ct) diamonds would be $3000. But if you want that same weight but only one diamond of a carat instead of two half carats the price per carat will increase substantially, because as the individual weight increases the diamond rarity increases as well. And a one carat F-VS2 diamond will have a price-per-carat of around 10,000$ per carat. That’s more than three time of a half carat diamond’s price-per-carat.
This comparison is valid when comparing diamonds which have same 4Cs values and other secondary influencers like fluorescence don't apply.