Most people would assume that the larger the diamond, the greater its worth, and this is indeed the case - provided, of course, that one is comparing like with like.

Diamonds differ greatly in colour and purity as well as size, and it can often be the case that two diamonds of the same weight differ widely in value.

Weight, then, is not everything when valuing a diamond, but it is clearly very important.

In order to weigh diamonds, dealers normally use very accurate weighing scales, which often have a digital readout to minimize the chances of error. The scales used are so sensitive that the weighing pan is usually enclosed inside a glass case so that the reading won't be affected by any draughts in the room. The weight is measured in carats.

So, what is a carat?

1 carat = 200mg (or 1/5th of a gram)

This means that five 1 carat diamonds would weigh one gram. We often express the weight of a diamond in terms of a fraction of a carat.

Another unit of measurement often used is the point. A point is one hundredth of a carat.

100 points = 1 carat

You could describe the weight of a diamond as 0.25 carats, or as 25 points; they mean the same thing.

When written down, carat is often abbreviated as ct, and point as pt.

So, the value of a stone can be based on its weight in carats or points. But we need to be able to be sure that we can tell accurately what its other attributes are, such as colour, clarity and cut.

Let's find out how to describe the colour of a diamond.