Carat (Weight / Diamond)
Also: Metric carat, ct, Carat weight, Brilliant carat
The metric carat (ct) is the legal unit of weight for diamonds and other gemstones and corresponds exactly to 0.2 grams.
The metric carat (abbreviation: ct) is the globally recognised unit of measurement for the weight of diamonds, coloured gemstones, and pearls. One carat equals exactly 0.2 grams or 200 milligrams. The unit traces back to the seeds of the carob tree (Ceratonia siliqua), whose kernels were used for centuries as a natural weight reference for gemstones due to their remarkably consistent mass. Since 1907, the metric carat has been internationally standardised and is today enshrined in law in virtually every country.
Important: The carat as a unit of weight (ct) is fundamentally different from the carat as a measure of gold fineness. The latter describes the gold content of an alloy and has nothing to do with weight.
Conversion and subdivision
The metric carat is subdivided into 100 points (English: points). The smallest value commonly traded is therefore 0.01 ct (1 point). This fine gradation is necessary because even minimal differences in weight can significantly affect the price of a stone.
1 ct = 0.2 g = 200 mg
0.5 ct = 0.1 g = 100 mg
0.01 ct (1 point) = 2 mg
1 g = 5 ct
| Carat weight | Grams | Milligrams | Common designation |
|---|---|---|---|
| 0.10 ct | 0.020 | 20 mg | 10-point stone |
| 0.25 ct | 0.050 | 50 mg | Quarter carat |
| 0.50 ct | 0.100 | 100 mg | Half carat |
| 1.00 ct | 0.200 | 200 mg | One carat |
| 2.00 ct | 0.400 | 400 mg | Two carat |
| 5.00 ct | 1.000 | 1,000 mg | Five carat |
The unit converter can be used to quickly convert carat weights to grams, milligrams, or other units of weight.
The four Cs – carat as an evaluation criterion
In the diamond trade, weight (carat weight) is one of the four central quality criteria, known as the "4 Cs":
- Carat – Weight (ct)
- Cut – Cut (proportion and symmetry)
- Colour – Colour (D to Z for diamonds)
- Clarity – Clarity (inclusions and blemishes)
Weight alone does not determine the price of a diamond. A flawless, well-cut 0.50 ct stone can be more valuable than a poorly cut 1.00 ct stone. Nevertheless, the price per carat rises sharply at so-called threshold values (0.50 ct, 1.00 ct, 2.00 ct), as demand for these "round" sizes is particularly high.
Carat for other gemstones and pearls
The metric carat applies not only to diamonds but to all gemstones – including rubies, emeralds, sapphires, and peridots – as well as pearls. Since different minerals have different densities, a 1 ct ruby has a different physical size from a 1 ct diamond, even though both weigh the same. For pearls, the carat is in some markets supplemented by the older Japanese unit of weight momme.
Distinction from other units of weight
Those familiar with precious metals may notice the apparent relationship with the troy weight system: one troy ounce (31.1035 g) corresponds to approximately 155.5 ct. However, the metric carat belongs to the SI system and is not part of the troy or avoirdupois system. It is used exclusively for gemstones and pearls, not for precious metal bars or coins. Those wishing to calculate the gold price or the melt value of jewellery calculate in grams or troy ounces.
In brief
The metric carat (ct = 0.2 g) is the precise, internationally standardised unit of weight for gemstones – not to be confused with the carat as a measure of gold fineness. Weight differences as small as one hundredth of a carat can noticeably shift the market value of a diamond.