Gemstone weights, including pearls & diamonds are expressed in carats. A carat weight is part of one of the 4C’s. While the purity of gold is measured in carats too, there’s a different meaning for precious metals and both measurements of carats shouldn’t  be confused.
The word "carat" is derived from the carob seed from which the unit is believed to be derived from. It is also thought that carob seeds had a naturally very tight tolerance in respect to their weight & size making it a suitable reference for small-scale weights. Although, recent research has found that the variation of carob seeds don’t differ from other seeds, taking away their believed superiority over other seeds.
In recent times, a carat has been defined to be 1/5 of a gram, or 0.2g. It’s divided into 100 sub-units known as points, with one point being 2mg or 0.002g. As the carat is linked to the kilogram, it is now accurately defined.
Establishing a diamonds weight is usually a straight-forward process: Place the Diamond on a suitable scale & note the displayed weight. This works solely for loose stones only and not for any gemstones set in jewellery.
When weighing a diamond on a scale it’s crucial that the scale has been calibrated. Once you are going into the realm of measuring minuscule quantities, a correct calibration is required. Scales used to weigh diamonds needs to read down to 1mg (= 0.001g), or AKA in the diamond trade: half a point.
Establishing the weight of a diamond set in jewellery will not be accurately possible. An estimation of the weight can be made by observing its size, i.e. diameter, of a brilliant cut diamond. For other cuts it is more guesswork. It’s important to note that estimates are not always accurate but sometimes it is the only way to get a rough idea of how big a stone is without disassembling a possibility very expensive piece of jewellery.
Obtaining the weight of a diamond is an easy task but as we just saw, one must appreciate the significance of doing so on a correctly calibrated scale and obtaining the weight by measuring a diamond's diameter should only be taken as an estimation.