A carat is the unit of weight by which a gemstone is measured. The carat-weight of a diamond is the easiest measurement of the 4C’s to determine. Carat-weight is not a factor which denotes the quality of a diamond, but merely its size by weight.
The word "carat" derives from the carob seeds that people used in ancient times to balance scales. So uniform in shape and weight are these little seeds that even today's sophisticated instruments cannot detect more than three one-thousandths of a difference between them.
"Carat” is not to be confused with "karat," which is used to indicate the purity of gold.
One carat is subdivided into 100 "points". Therefore a diamond measuring 75 points is 3/4 carat in weight, or 0.75ct. There are five carats in a gram.
The size of a diamond is proportional to the carat-weight of a diamond. When rough diamonds are cut and polished into finished diamonds, as much as 2/3 of the total carat-weight may be lost. Since larger rough gems of high quality are found less frequently than smaller rough gems of high quality, a single two carat diamond will be more expensive than 2 one carat diamonds of the same quality. However, two diamonds of equal carat-weight can differ greatly in value due to their cut, color, and clarity.
Carat-weight by itself will not reflect a diamond’s size. Instead, two other factors must be considered:
Distance across the top of the diamond measured in millimetres
Since a diamond is viewed from top when set into a ring or pendant, it is essential to determine the distance across the top of a diamond. A deeply cut diamond has a greater proportion of its total weight "hidden" in the depth, resulting in a smaller diameter than a well cut diamond.
The cut grade of a diamond
The amount of light reflected from a diamond depends on its cut grade. Hence, when a diamond is well cut, maximum light is reflected from the top and makes it appear larger. It is therefore possible to get a diamond of lower carat but with higher cut grade and vice versa.
Carat is abbreviated to "ct" or "CT" when describing a single stone. "TCW," meaning "total carat weight," is added when jewellery is set with multiple diamonds.
It has to be noted that the carat weight does not refer to exactly one figure. It comprises of stones within a certain weight range. For example:
1 carat - Stones in weight range of 0.95 ct to 1.05 ct
0.75 carat - Stones in weight range of 0.72 ct to 0.76 ct
0.50 carat - Stones in weight range of 0.47 ct to 0.56 ct
0.25 carat - Stones in weight range of 0.23 ct to 0.26 ct
This holds true for all carat weight figures.
What Carat-Weight Is Right For Me?
This question has no direct answer. It really depends. Differences in size are clearly visible even to the untrained eye, and weight clearly does matter because it influences cost significantly.
However, size alone does not guarantee beauty. The quality of a diamond in cut, grade, and diameter should also be considered.
In deciding the importance of carat weight, it is vital to take into account the recipient's preferences. If the dream is a one carat diamond, even the most beautiful half carat stone will be a disappointment. If a large carat weight is an important factor, but there is a strict budget, consider a diamond with a good cut, SI1-SI2 clarity, and an I or J colour grade. This will make room for a higher carat weight.
If quality is the primary factor, once the cut, colour, and clarity grade have been selected, it is easy to determine the carat-weight of the diamond that will fit within your budget.
Diamond prices jump at the full- and half-carat weights. Diamonds just below these weights cost significantly less, and, because carat weight is distributed across the entirety of the diamond, small size differences are almost impossible to detect visually.
Remember that slender fingers make small diamonds look bigger.
Think about what sort of setting will hold the diamond. The setting you choose must be able to fit the carat-weight of your diamond.