Carat weight

Carat weight is the mass of a stone. A metric carat is defined as 200 milligrams, which is the value that the US National Institute of Standards and Technology lists for the unit. Each carat splits into 100 points, so one point is 0.01 ct, or 2 mg.

Jewellers describe small stones in points: GIA gives a 0.25 ct diamond as a “twenty-five pointer”. The word carat comes from carob seeds, which early gem traders used as counterweights on their scales. Weight is read from a calibrated digital scale, which makes carat weight the most objective of the 4Cs.

Rings with several stones are often described by total carat weight, which is the combined weight of all the stones, not the size of any one of them.

Weight tracks volume, and volume grows faster than width. If two round diamonds have the same proportions, doubling the weight makes the stone about 26% wider, not 100% wider. The face you see from above gains about 59% in area. That is why the step from 1.00 ct to 2.00 ct is noticeable but nothing like a doubling, and why a 0.99 ct stone is hard to tell from a 1.00 ct one by eye.

The table shows the width of a stone relative to a 1.00 ct stone, assuming identical proportions. These figures are arithmetic from the cube-root relationship, not measurements of particular diamonds.

No. Weight that sits deep in the stone adds to the total without adding to the face. If a diamond is improperly cut, the added weight may do nothing except reduce its brilliance, and a smaller stone that is well cut may look larger to the naked eye than a poorly cut bigger one. GIA's cut grade includes weight ratio, which compares a stone's weight with its diameter, so cut and carat weight are linked.

Shape matters just as much. Elongated shapes such as an oval, marquise or pear can give the illusion of greater size than a round diamond of the same carat weight. Ask for the measurements in millimetres as well as the carat weight, and compare those, especially when you compare two shapes.

Setting also changes the impression. A halo can make the centre stone appear larger, though it adds nothing to that stone's carat weight.

Buyers search for round numbers, and prices tend to step up at them. GIA calls 1.00, 1.50 and 2.00 carats “magic sizes” for their desirability, and notes that it is hard to tell a 0.99 ct diamond from a 1.00 ct one by eye, yet the price difference can be significant.

Price is also not proportional to weight. With colour, clarity and cut equal, a 1 ct diamond costs more than twice as much as a ½ ct diamond. GIA's explanation is that larger diamonds are more rare and more desirable.

The honest trade-off: a stone just under a round number looks almost the same on the hand, so it may stretch a budget further. A stone at the round number may suit you better if the number itself matters to you. Neither choice is wrong. Two diamonds of the same weight can also differ greatly in value because of their colour, clarity and cut.

They sound the same and are unrelated. A carat, with a C, is a unit of mass for gemstones, including diamonds. A karat, with a K, describes the purity of gold, for example in 14K gold. A 14K ring can hold a 1 carat diamond, and the two numbers say nothing about each other. The full explanation of the gold measure is in the entry on karat.

Start with the size you want to see on the hand, then work backwards to weight, and let the shape, cut and setting do part of the job. A well-cut stone in a shape that spreads, such as an oval, can look larger than a round stone of the same weight, and that can free budget for better colour or clarity.

If you are designing a custom engagement ring, the design studio is a good place to note the shape and size you are drawn to before you talk to a jeweller. You can also browse engagement rings to see how different stone sizes sit on a finger.