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The first and easiest of the 4C’s to define, the Carat Weight of the diamond. The carat weight measures the mass of a diamond. One carat is defined as 200 milligrams. The point unit—equal to one one-hundredth of a carat (0.01 carat, or 2 mg)—is commonly used for diamonds of less than one carat. All else being equal, the price per carat increases with carat weight, since larger diamonds are both rarer and more desirable for use as gemstones.

Carat weight scale The price per carat does not increase smoothly with increasing size. Instead, there are sharp jumps around milestone carat weights, as demand is much higher for diamonds weighing just more than a milestone than for those weighing just less. As an example, a 0.95 carat diamond may have a significantly lower price per carat than a comparable 1.05 carat diamond, because of differences in demand and due to the rarity of finding gem quality diamonds in larger sizes.

Over 120,000,000 carats of diamonds are mined each year of which only around 20% or 24,000,000 carats are of gem quality, of this only around 6,000,000 carats are larger then 1carat each. Diamond are sorted into over 14,000 different bands which makes each diamond unique as to its 4C’s.

Total carat weight (t.c.w.) is a phrase used to describe the total mass of diamonds or other gemstone in a piece of jewellery, when more than one gemstone is used. Diamond solitaire earrings, for example, are usually quoted in t.c.w. when placed for sale, indicating the mass of the diamonds in both earrings and not each individual diamond. T.c.w. is also widely used for diamond necklaces, bracelets and other similar jewellery pieces.