Carat (Weight / Diamond)
Also: Metric carat, ct, Carat weight, Diamond carat
The metric carat (ct) is the legal unit of weight for diamonds and other gemstones and equals exactly 0.2 grams.
The metric carat (abbreviated ct) is the worldwide unit of measure for the weight of diamonds, coloured gemstones and pearls. One carat equals precisely 0.2 grams, or 200 milligrams. The unit traces back to the seeds of the carob tree (Ceratonia siliqua), whose kernels, thanks to their remarkably uniform mass, served for centuries as a natural weight reference for gemstones. Since 1907 the metric carat has been standardised internationally and is today enshrined in law in almost every country.
Important: the carat as a unit of weight (ct) is fundamentally different from the carat as a fineness statement for gold. The latter describes the gold content of an alloy and has nothing to do with weight.
Conversion and subdivision
The metric carat is divided into 100 points. The smallest value ordinarily seen in trade is therefore 0.01 ct (1 point). This fine gradation is necessary because even tiny weight differences can markedly 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 | Typical name |
|---|---|---|---|
| 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 |
With the unit converter you can quickly turn carat weights into grams, milligrams or other units of weight.
The four Cs - carat as a valuation 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 - proportion and symmetry
- Colour - hue (D to Z for diamonds)
- Clarity - inclusions and blemishes
Weight alone does not determine the price of a diamond. A flawless, well-cut 0.50-ct stone can be worth more than a poorly cut 1.00-ct stone. Even so, the price per carat jumps sharply at so-called threshold values (0.50 ct, 1.00 ct, 2.00 ct), because demand for these "round" sizes is especially 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. Because different minerals have different densities, a 1-ct ruby is a different physical size from a 1-ct diamond, even though both weigh the same. For pearls, in some markets the carat is still supplemented by the older Japanese weight measure, the momme.
Distinguishing it from other units of weight
Anyone familiar with precious metals may stumble over an apparent kinship with the troy weight system: one troy ounce (31.1035 g) equals about 155.5 ct. The metric carat, however, belongs to the SI system and is not part of the troy or avoirdupois systems. It is used exclusively for gemstones and pearls, not for precious-metal bars or coins. If instead you want to calculate the gold price or the melt value of jewellery, you work in grams or troy ounces.
Key takeaway
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 gold-fineness term. Even weight differences in the hundredths of a carat can noticeably shift the market value of a diamond.