"Carat" or "carat weight" is a measure of how much a diamond weighs and is called ct. One carat actually represents one-fifth of a gram, or 200 mg. The carat can be further divided into 100 "points" and one point equals 0.01 ct. This allows very accurate measurement to two decimal places. The jeweler can thus describe a stone that weighs less than 1 ct, e.g. as 0.25 ct, or 25 points. Carat weights above 1 ct are expressed in carats in two decimal places.
The carat weight of a diamond is closely related to its size, yet it is a different quantity. In addition, there are other aspects that can affect the size of the stone that can be seen by the eye, the most important of which are the cut and the shape. If you look at the stone only with the naked eye, your impression will be affected first by its proportions rather than the carat weight itself.
How a diamond looks is affected by its shape. For example, coated designs (marquise, oval) optically enlarge the diamond. Nevertheless, two stones of the same shape and carat weight may differ in other proportions. The depth and dimensions of the board must also be taken into account. The depth given in % expresses the ratio between the total depth of the stone and its diameter. Deep-cut stone has a larger proportion of the total carat weight hidden in the depth of the pavilion (thus it has a higher depth), which leads to a smaller diameter than with a perfectly cut diamond. The diamond with the lowest depth will look bigger. The appearance of a diamond and its size are, in addition, also influenced by the setting of the stone in the jewelry.