In addition to color, clarity and cut, weight provides a further basis in the valuation of a diamond. The weight of diamonds, and all other gemstones, is measured in carats. Smaller diamonds are more readily available than larger ones, which results in higher values based on weight.
The weight or overall size of a diamond is measured in carats. In fact, all gemstones are measured in this fashion. Of course, the larger the diamond, the more you can expect to pay for it, however the price does not increase on an even scale. A 2 carat diamond will not be twice the cost of a 1 carat diamond, despite being twice the size. The larger the diamond, the rarer it becomes and as such the price increases exponentially.
When choosing a diamond for yourself or someone else, keep in mind that the same size diamond can look bigger depending on the type of setting it is mounted in. Also, a 1 carat stone will look bigger on a person with a size 6 finger, than it will on a person with a size 8 finger.