The carat is the world standard for measurement of weight of a diamond.
The term “carat” goes back to the traders of the ancient world. A standard weight was required for precious gems as merchants of the ancient Mediterranean and Middle East were dependent on the ability to trade with a reasonably consistent unit of measurement. It was this need that lead to the adoption of seeds and grains as widespread accepted units of measurement. The present day Metric-Carat is 200 milligrams.
If the recipient’s heart is set on a certain size diamond, then carat weight will probably be the most important factor in your search until the desired size is attained. At that point, other criteria will take on more importance. Most women can tell you the carat weight and shape of their ideal diamond, and most men can tell you the price.