The most important thing to understand about carat weight is that the weight of the diamond isn't everything, you must also consider the quality, clarity, colour and cut.
Carat is the unit of weight measurement of a diamond. Originating from the remarkably constant weight of a carob seed, one carat equals 200mg or 1/5 of a gram (0.007 of an ounce). Each carat is further divided into 100 points.
Carat weight of an unset diamond is determined using a scale, which is capable of weighing to 0.002 carat. The carat weight of a diamond set in a piece of jewellery is determined using measurements and a volume formula, and is therefore not as accurate as actually weighing the diamond. Be sure you know the exact weight of your diamond as it can have a significant effect on value.