A diamond's weight is measured in what is known as a 'carat', which is a small unit of measurement equal to 200 milligrams. Carat is not a measure of a diamond's size, since cutting a diamond to different proportions can affect its weight. (The word 'Karat' is used to express the purity of gold, and is not used in relation to diamonds.) Here is a diagram that shows the relative size to carat weights:

The most important thing to remember when it comes to a diamond's carat weight is that it is not the only factor that determines a diamond's value. In other words, bigger does not necessarily mean better. All four Cs - Cut, Colour, Clarity and Carat Weight must be balanced in order to arrive at a diamond that fits your budget. None of the 4Cs is mutually exclusive, nor is any one more important than the others.

The word carat actually comes from the word carob (as in carob seeds), which is how ancient cultures measured the weight of diamonds on their scales. In 1913, however, the weight was standardized internationally and adapted to the metric system.

Although they can be measured when mounted in jewellery, diamonds are most accurately weighed when they are not mounted in a setting. In fact, gemological laboratories will only grade diamonds that are unmounted. A diamond grading report will tell you the exact carat weight, to the nearest hundredth of a carat, for that particular diamond. Each carat is divided into 100 parts called 'points.' So a 1-carat diamond has 100 points, a half-carat has 50 points, etc.

The carat weight of a diamond is an extremely important determining factor in its value. Diamonds are valued on a per-carat basis. Diamond values increase disproportionately as the size of the stone increases. In other words, a two-carat stone will not necessarily cost twice per carat than a one-carat stone. It could cost much more, since diamonds are rarer in larger sizes. As you take a stone of a particular cut, clarity and colour and move its carat weight to the next price category, you may see quite a large increase in the price per carat.

Remember that size isn't everything. When choosing a diamond, all 4Cs must be taken into account. The key is to strike a balance among them, while still working within your budget.