Diamond carat weight (GIA)
The fourth C, and the one that is measured rather than graded. There is no "excellent" carat weight — unlike clarity, colour and cut, carat has no scale positions, only a number. It also carries the sharpest price cliff in the category.
| Property | Value | Authority |
|---|---|---|
| What it measures | Weight, not size. A metric carat is defined as 200 milligrams, subdivided into 100 points, allowing measurement to the hundredth decimal place — and to the thousandth for rounding. [1] Two diamonds of equal carat weight can look different sizes: cut proportions decide how much of the weight sits where it can be seen. | — |
| Applies to | Diamonds and, as a unit of weight, gemstones generally. [1] | — |
| Viewing condition | Measured on a calibrated scale, not assessed by eye. [1] This is why carat is not a grade. The other three Cs place a stone on a scale of named positions; carat returns a number. | — |
| Grades in full | No grades — a continuous measurement. Trade usage: a 0.25 carat stone is a "twenty-five pointer"; above one carat, weights are spoken in carats and decimals ("one point oh eight carats"). GIA identifies 1.00, 1.50 and 2.00 carats as "magic sizes" whose desirability creates a price step. A 0.99 carat diamond is difficult to distinguish visually from a 1.00, yet the price difference can be significant — which makes buying just below a round weight the most actionable saving in the 4Cs. [1] | — |
Sources
- GIA — Diamond Carat Weight — Gemological Institute of America (GIA 4Cs, Diamond Carat Weight)