A lot of people are unsure what a carat is when it comes to choosing a stone for their piece of jewellery. It is actually a measurement of weight, not size and it is used for ALL gemstones, not only diamonds! The carat weight of a stone does determine the size of the stone, but it is not necessarily in the measurement of the diameter of the table of the stone, it could be the depth of the stone that is holding all the weight.
One carat weighs 200 milligrams which is 1/5 of a gram and it is divided into 100 points. To see how much your stone weighs it would be written in the format 0.25ct and this would mean your stone would weigh a quarter of a carat. But it is important to remember; if two stones are the same size they can have very different values because there are other qualities to factor in such as; clarity, cut and colour.
There is a little bit more about the other qualities of a stone on the article below: