As with all precious stones, the weight and size of a diamond is expressed in "carats". The word carat originated from a natural unit of weight, the "carob seed". Carob seeds were used as a gem weight in ancient times. Diamonds were traditionally weighed against these seeds until the system was standardized and one carat was fixed at 0.20 grams (1/5 of a gram).

Because of the need for such precision due to the fact that a fraction of a carat could mean hundreds of dollars, diamond weight is measured to a thousandth of a carat and rounded to the nearest hundredth or "point". A one carat diamond is divided into 100 points, a half carat diamond weighs 50 points and a quarter carat diamond is 25 points.

A diamond that weighs one carat will be much more valuable than a one carat total weight cluster of diamonds. Larger diamonds are much more rare and therefore more valuable and costly. As the weight of a diamond increases, so will the price.

Gemologist determining the exact carat weight of a diamond.

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