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Carat is the measuring unit of diamond weight. A karat definition has changed over time, but since 1913 the international standard has been 200 milligrams or 1/5 of a gram. One karat is divided into 100 parts, called “ points.” So, a 25-point diamond weighs a quarter karat, and a 50-point diamond weighs a half karat. Therefore, a diamond may be referred to as a 3/4-carat or 75-point stone.
Jewelers usually express diamond weights in carats and decimals. A 1.09-carat stone, for example, is said to weigh “one oh nine carats,” while a 0.99-carat diamond weighs “ninety-nine points.”
“Carat total weight” (Ct. T.W.) describes the combined total weight of all the stones in a piece of jewelry having more than one diamond.
As diamonds are mined, larger stones are found much less frequently than smaller ones. This makes large diamonds much more valuable. The rarer the diamond, the greater it is worth it. In fact, diamond prices tend to rise exponentially with carat weight. A larger stone costs more per carat. So, all other factors being equal, a 0.50-Ct. Diamond will cost more than twice that of a 0.25-Ct.; a single 2-carat diamond will cost much more than two 1-carat diamonds of the same quality. Also, you will pay a premium for stones that are above a full carat weight. For example, a .95-carat diamond will cost somewhat more than a .90-carat stone, but a 1-carat stone will cost significantly more than a .95-carat diamond.
Bigger is not always better. A diamond's ultimate value is based on a balance between the 4C's: cut, clarity, color, and carat weight. None of these factors is automatically more important than the others. Instead, all of them must be considered together to determine the true worth of a diamond.