Does size matter ?
Carat weight is not a factor which denotes diamond quality. It purely denotes the size of a diamond, by its weight. Obviously it also affects the value or price of a diamond, because consumers tend to prefer bigger rather than smaller diamonds. Large diamonds are also rarer than small diamonds. Because there is higher demand and lower supply for large diamonds, they command higher prices. Often people talk about the size of diamonds. Size in this context really means weight, as large diamonds will normally weigh more than small ones.
What is a carat ?
Historically the carat is supposed to have been derived from the weight of locust bean or carob bean seeds, from the Greek Keraton. Until earlier this century, there were at least two different standards for the carat. In 1914, the carat, or metric carat as it was known, was defined as 1/5th of a gram.
It is not always easy to establish the weight of a mounted diamond, it would need to be removed from its mount, and then reset, which always involves some slight risk, not to mention time and expense.
For round diamonds it is fairly easy to estimate their weight by measuring their diameter and, preferably, their depth. If this is done accurately, the weight can be estimated within a fine tolerance (about 1 or 2 %). If the depth cannot be measured, the weight can still be estimated from the diameter, but with less precision.
For diamonds of other shapes, it is more difficult to estimate the weight, but specialised diamond gauges such as the Leveridge gauge come with a booklet of conversion tables, and instructions (albeit rather unmathematical) for calculating diamond weights.
Weight / diameter table
The following table gives an approximate guide to the relative diameter size in millimetres, and weight of modern round brilliant cut diamonds of ideal proportions.