Carat - Weight Unit

Weights of gemstones, including diamonds, and pearls are expressed in the units of carats, and Carat Weight is one of the famous 4Cs. While the purity of gold is measured in carats also, it has a different meaning for precious metals and the two measurements of carats should not be confused.

The word "carat" is derived from the carob seed from which the unit is believed to be derived. It was thought that carob seeds had a naturally very tight tolerance with respect to their size and weight, making it an ideal reference for small weights. However, some recent research found that the variation of carob seeds are not too different from any other seed, making them not superior to other seeds.

In modern times, the carat has been defined to be 1/5 of a gram, or 0.2g. It is divided into 100 sub-units called points, with one point being 2mg or 0.002g. As the carat is linked to the kilogram, it is now accurately defined. [Strictly speaking the exact definition of a kilogram is something that still troubles modern science but it is nothing that will ever change daily life and definitely will not alter the unit of a carat].

Establishing the weight of a diamond is usually a straight-forward process: Place the stone on a suitable scale and read off the weight. This works for loose stones only and not for diamonds set in jewellery.

When weighing a diamond on a scale it is important that the scale has been calibrated correctly. Once you are going into the realm of measuring tiny quantities, proper calibration is a must. A scale used to weigh diamonds accurately has to read down to 1mg (= 0.001g), or as expressed in the diamond trade: half a point.

Establishing the weight of a diamond set in jewellery is not accurately possible. One can estimate the weight by the size, i.e. diameter, of a brilliant cut diamond. For other cuts it is more guesswork than anything else. It is important to note that such estimates are by no means very accurate but sometimes it is the only way to get an idea of how big a stone is without disassembling a potentially very expensive piece of jewellery.

Getting the weight of a diamond is a relatively easy task but as we just saw, one must appreciate the importance of doing so on a calibrated scale and that obtaining the weight by measuring a diamond's diameter is only an estimate.

Further Reading

This covers the discussion of the Carat as a weight unit. From here, you can read more about the other Cs of a diamond or go back to an overview of the 4Cs by clicking on the links below: