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 valid unit of measurement for the weight of diamonds, coloured gemstones and pearls. One carat corresponds exactly to 0.2 grams or 200 milligrams. The unit traces back to the seeds of the carob tree (Ceratonia siliqua), whose kernels served for centuries as a natural weight reference for gemstones because of their remarkably uniform mass. Since 1907 the metric carat has been internationally standardised and is today enshrined in law in almost all countries.
Important: The carat as a unit of weight (ct) is fundamentally different from the carat as a fineness measure 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 subdivided into 100 points (points). The smallest value common in the trade is therefore 0.01 ct (1 point). This fine gradation is necessary because even minimal weight differences considerably 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 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 |
With the unit converter you can quickly convert 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 - 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 jumps at so-called threshold values (0.50 ct, 1.00 ct, 2.00 ct), because 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 to pearls. Because 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 still complemented by the older Japanese weight measure momme.
Distinction from other units of weight
Anyone who knows precious metals stumbles over the apparent relationship with the troy weight system: one troy ounce (31.1035 g) corresponds to about 155.5 ct. The metric carat, however, 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. Anyone wishing to calculate the gold price or the melt value of jewellery, by contrast, works 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 hundredth-carat range can noticeably shift the market value of a diamond.