A carat is made up of 100 points, so a half-carat diamond is also known as a 50 point diamond. Although used most famously for diamond jewellery, carat is also used as a standard measure for other gemstones. However, some gemstones are denser than others so different gems of the same weight are not necessarily the same size.

Larger diamonds normally have a higher value as they are rare and therefore in more demand for diamond jewellery than smaller diamonds of the same quality. Of course, a diamond being large does not automatically qualify it as valuable, as the quality of the cut, clarity and colour all need to be taken into consideration.

A large stone with poor cut, clarity and colour will not be worth any more simply because of its size. However, a large stone with excellent cut, clarity and colour will be worth more than smaller diamonds of equally good quality. For example, diamond rings consisting of one large two carat diamond will nearly always be more expensive than diamond rings made up of several smaller diamonds which may total two carats between them.

Back in the early days of diamond trading for jewellery, the stones were measured against the weight of carob seeds, which are remarkably uniform in size and weight. One carob seed came to be one carat. There is a finer measurement called ‘point’. One point is one hundredth of a carat, so a half carat diamond is a fifty point diamond.

While size certainly does matter, diamond quality is also important. Clarity and colour will also affect the price and desirability of a given stone. So will the type of cut.

One carat is 0.2g. That doesn’t sound like much, but one carat diamond engagement rings are actually quite good and the central stone in question will probably look quite impressive. Most of the stones used in diamond jewellery are smaller than that. A 2 carat diamond is on the decidedly large side, and a 3 carat diamond is truly outstanding in size. Good quality loose stones of that weight are usually sold for tens of thousands of pounds when cut, or more.