Was ist ein/eine Carat (ct)?
Formal Definition
The carat (symbol: ct) is a unit of mass equal to exactly 200 milligrams (0.2 grams, or 0.00020 kilograms). It is the standard unit of measurement for gemstones and pearls worldwide. The metric carat was adopted by the Fourth General Conference on Weights and Measures (CGPM) in 1907 and has since been universally accepted in the international gem and jewelry trade.
The carat should not be confused with the karat (symbol: K or kt), which is a measure of gold purity. In American English, "carat" refers to gemstone weight and "karat" to gold purity. In British English and most other languages, "carat" is used for both, though context makes the meaning clear. The abbreviation "ct" always refers to gemstone weight, while "K" or "kt" always refers to gold purity.
Precision in the Gem Trade
In the gem trade, carat weight is measured with extraordinary precision. Gemstones are weighed to the nearest hundredth of a carat (0.01 ct = 2 mg) using high-precision analytical balances. For diamonds, even thousandths of a carat can affect value. A diamond's weight is typically reported to two decimal places, and gemological laboratories such as the GIA (Gemological Institute of America) certify weights to the hundredth of a carat. Since diamond prices increase nonlinearly with carat weight — a 1.00 ct diamond costs significantly more per carat than a 0.99 ct stone — precise measurement at weight boundaries has enormous commercial significance.
Etymology
The Carob Seed
The word "carat" derives from the Greek "keration" (κεράτιον), meaning "carob seed" or "little horn" (from "keras," horn, referring to the shape of the carob pod). Carob seeds (Ceratonia siliqua) were used as balance weights in ancient gem and gold trading across the Mediterranean, Middle East, and India because they were believed to have remarkably uniform weight.
The Arabic form "qirat" (قيراط) was adopted from the Greek and became the standard term in Islamic commerce, which dominated the gem trade routes between India and Europe for centuries. The Italian "carato" and French "carat" evolved from the Arabic. The word entered Middle English in the 15th century.
The Uniformity Myth
Modern botanical research has shown that carob seeds are not actually as uniform in weight as ancient traders believed. Studies have found that carob seeds vary by about 5% in mass (standard deviation approximately 12 mg for seeds averaging 200 mg). However, recent research also suggests that carob seeds are more uniform than the seeds of other common plants, which may explain why they were preferred as reference weights. The 200 mg value of the modern metric carat corresponds closely to the average mass of a carob seed, confirming that the historical connection is genuine.
Precise Definition
The Metric Carat
The metric carat is defined as exactly 200 milligrams (0.2 g or 2 × 10⁻⁴ kg). This definition was first proposed at the Fourth General Conference on Weights and Measures in 1907 and was adopted internationally over the following decades. The standardization replaced numerous national and regional carat definitions that had varied between 187 and 216 milligrams.
International Adoption
The metric carat of 200 mg was adopted by different countries at different times: France in 1907, the United States and the United Kingdom in 1913, and most other countries by the 1930s. By mid-20th century, the metric carat was universally accepted. The choice of 200 mg — a round number in the metric system — was a deliberate simplification that made the new carat easy to convert to grams and milligrams while remaining close to the average of the historical carat values.
Points and Grains
The carat is subdivided into 100 points: 1 ct = 100 points. A "10-point diamond" weighs 0.10 ct or 20 mg. This system is used routinely in the diamond trade for stones under one carat. The point system provides a convenient way to express small diamond weights without using decimal fractions of a carat. Historically, the carat was also divided into four "diamond grains" (1 grain = 0.25 ct = 50 mg), but the point system has almost entirely replaced grain measurement in modern trade.
Geschichte
Ancient Origins
The use of seeds as balance weights for precious stones and metals dates to antiquity. Carob seeds were used in gem trading in ancient Greece, Rome, and the Middle East. Indian gem merchants used the ratti seed (Abrus precatorius, also called the gunja seed) as a reference weight, and the ratti was roughly similar in mass to the carob seed. Archaeological evidence suggests that seed-based weight systems were in use by 2000 BCE in the Indus Valley civilization.
The carob-seed carat became the dominant weight standard for gemstones along the trade routes connecting India, the Arabian Peninsula, and Europe. Arab and Jewish merchants who controlled much of the medieval gem trade used the qirat as their standard, and European merchants adopted it through the Italian trading cities of Venice, Genoa, and Florence.
Pre-Metric Chaos
Before the metric carat was standardized, the carat varied significantly by location. The Amsterdam carat (used in the world's most important diamond trading center) was approximately 205.3 mg. The Florence carat was about 197.2 mg. The Leipzig carat was approximately 205.4 mg. The London Board of Trade carat was 205.3 mg. The Lisbon carat was approximately 205.8 mg. These variations, though small in absolute terms, became increasingly problematic as the international gem trade grew and high-value stones were traded across borders.
Standardization
The push for standardization came from the diamond and gem industries themselves. In 1871, the Syndical Chamber of Diamond Dealers in Paris proposed a standard metric carat of 200 mg. The proposal was debated for decades, with resistance from some trading centers that benefited from the ambiguity of different carat standards. Finally, in 1907, the Fourth CGPM adopted the metric carat of 200 mg. France adopted it immediately, and other countries followed: the United States in 1913, the United Kingdom in 1913, Belgium in 1913, the Netherlands in 1911, and Japan in 1920.
The transition was remarkably smooth compared to other measurement standardizations, partly because the gem industry was relatively small and internationally connected, and partly because dealers recognized that a single standard would simplify trade and reduce disputes.
Aktuelle Verwendung
The Diamond Trade
The carat is the universal unit of weight in the global gem and jewelry trade. Every diamond, ruby, sapphire, emerald, and other precious or semi-precious stone sold commercially is weighed in carats. The Kimberley Process, which certifies rough diamonds, uses carats. Diamond exchanges in Antwerp, Mumbai, Tel Aviv, New York, and Hong Kong all trade in carats. The Rapaport Diamond Report, the industry's standard price list, quotes per-carat prices for different combinations of cut, color, clarity, and carat weight.
Gemological Laboratories
Gemological laboratories — including the GIA (Gemological Institute of America), AGS (American Gem Society), and HRD Antwerp — weigh and certify gemstones in carats to the hundredth (0.01 ct). For diamonds, the "Four Cs" system (Cut, Color, Clarity, and Carat weight) is the universal grading framework, and carat weight is the most objectively measurable of the four qualities.
Pearl Trade
Pearls are also measured in carats, though historical pearl weights used the grain (1 grain = 0.25 ct). Modern pearl trading has largely adopted the carat, with some dealers still referencing grains for traditional or auction purposes. The most valuable pearls can weigh several hundred carats.
Mining and Production
Diamond production statistics are reported in carats. Global rough diamond production is approximately 120 to 130 million carats per year. Russia is the world's largest diamond producer by volume (approximately 32 million carats per year), followed by Botswana, the Democratic Republic of Congo, and Australia. The value per carat varies enormously: gem-quality diamonds may be worth thousands of dollars per carat, while industrial diamonds may be worth only a few dollars per carat.
Everyday Use
Buying Jewelry
For most people, the carat is encountered when purchasing diamond jewelry. Engagement rings in the United States average approximately 1.0 to 1.2 carats for the center stone. A one-carat round brilliant diamond has a diameter of approximately 6.5 mm. Consumers quickly learn that diamond prices increase dramatically with size: a 2-carat diamond costs far more than twice the price of a 1-carat diamond of the same quality, because larger diamonds are exponentially rarer.
Popular Size Categories
In the jewelry market, certain carat weights are psychologically significant. Diamonds at "magic numbers" — 0.50 ct, 0.75 ct, 1.00 ct, 1.50 ct, and 2.00 ct — command premium prices per carat. A 1.01 ct diamond may cost 15-20% more per carat than a 0.99 ct stone of identical quality. This discontinuous pricing creates a strong incentive for cutters to maximize carat weight even at the expense of optimal proportions.
Understanding Total Carat Weight
Jewelry marketed with total carat weight (TCW or CTW) sums the weights of all stones in a piece. A ring described as "2 CTW" might contain one 1-carat center stone and 20 smaller accent stones totaling another carat. Understanding this distinction is essential for informed jewelry purchases.
Colored Gemstones
For colored gemstones (rubies, sapphires, emeralds, and others), carat weight has a different visual relationship to size than for diamonds, because different gemstone species have different densities. A 1-carat ruby (density 4.0 g/cm³) appears smaller than a 1-carat diamond (density 3.52 g/cm³), which appears smaller than a 1-carat emerald (density 2.76 g/cm³). Consumers shopping for colored stones should compare by millimeter dimensions rather than carat weight alone.
In Science & Industry
Gemology
In gemology — the scientific study of gems — the carat is the fundamental unit of mass measurement. Gemological research on crystal growth, optical properties, and geological formation uses carat weight as a standard parameter. Studies of diamond inclusions, crystallographic orientation, and spectroscopic properties routinely report specimen weights in carats.
Material Science
In material science, synthetic diamond production is measured in carats. Chemical vapor deposition (CVD) and high-pressure high-temperature (HPHT) methods produce synthetic diamonds for industrial and gem applications. Laboratory-grown gem diamonds now reach sizes of 10+ carats, and their production statistics are reported in carats. Industrial diamond production (for cutting, grinding, and drilling tools) is measured in carats and metric carats (thousands of carats).
Geology
In geology, particularly in the study of kimberlite pipes and alluvial deposits, diamond concentration in ore is expressed as carats per hundred tonnes (cpht). A commercially viable kimberlite pipe might have a grade of 30 to 200 cpht. This metric is fundamental to the economic evaluation of diamond deposits worldwide.
Pharmacology
While the carat is not used in pharmacology directly, the underlying mass (200 mg = 0.2 g) falls in the range of common pharmaceutical dosages. Some historical pharmaceutical texts reference carat weight for small quantities of precious or exotic ingredients, though this practice ended centuries ago.
Interesting Facts
The word 'carat' comes from the Greek 'keration,' meaning carob seed. Ancient gem traders used carob seeds as balance weights, believing them to be remarkably uniform in mass. Modern studies show they average about 200 mg — the exact value chosen for the metric carat.
The most expensive diamond ever sold at auction is the Blue Moon of Josephine, a 12.03-carat fancy vivid blue diamond that sold for $48.5 million in 2015 — approximately $4.03 million per carat.
The Cullinan Diamond, the largest gem-quality diamond ever found, weighed 3,106.75 carats (621.35 grams) in its rough state when discovered in South Africa in 1905. It was cut into nine major stones and 96 smaller brilliants.
A one-carat round brilliant diamond has a diameter of approximately 6.5 mm — about the size of the eraser on a standard pencil. A five-carat round diamond is about 11 mm in diameter.
Diamond prices increase nonlinearly with carat weight. A 1.00 ct diamond of a given quality may cost 15-20% more per carat than a 0.99 ct stone of identical quality, because 'magic number' sizes command a premium.
Global rough diamond production is approximately 120-130 million carats per year, but only about 20-30% of mined diamonds are gem quality. The rest are used for industrial purposes such as cutting, grinding, and drilling.
The metric carat (200 mg) was standardized in 1907, replacing dozens of different regional carats that varied from 187 to 216 mg. The Amsterdam carat was 205.3 mg, the Florence carat was 197.2 mg.
Synthetic diamonds produced by HPHT (high-pressure high-temperature) and CVD (chemical vapor deposition) methods now account for the majority of industrial diamonds and a growing share of the gem market. Lab-grown gems over 10 carats have been produced.
The Hope Diamond, one of the most famous gems in the world, weighs 45.52 carats and is displayed at the Smithsonian National Museum of Natural History. Its deep blue color comes from trace amounts of boron in the crystal lattice.
A carat is subdivided into 100 'points.' A '25-point diamond' weighs 0.25 carats (50 mg). The point system allows jewelers to express small diamond weights without using decimal fractions.
Regional Variations
Universal Standard
The metric carat of 200 mg is used universally worldwide. Unlike most measurement units, which have regional variations, the carat has been globally standardized since the early 20th century. Diamond and gem trading centers in Antwerp (Belgium), Mumbai (India), Tel Aviv (Israel), New York (USA), and Hong Kong (China) all use the identical metric carat.
Spelling Variations
The only significant regional variation is in spelling. In American English, 'carat' refers to gemstone weight and 'karat' to gold purity. In British English, Australian English, and most other varieties of English, 'carat' is used for both. In French, it is 'carat'; in German, 'Karat'; in Spanish, 'quilate'; in Italian, 'carato.' The abbreviation 'ct' for gemstone weight is universal.
Historical Variations
Before 1907, carats varied by region. The Amsterdam carat (205.3 mg), the Florence carat (197.2 mg), the London Board of Trade carat (205.3 mg), and the Lisbon carat (205.8 mg) all differed slightly. These variations occasionally surface in historical contexts — antique gemstone descriptions may reference pre-metric carat weights that differ from modern measurements by 1-8%.
India and Traditional Systems
In India, the ratti (or rati) was traditionally used alongside the carat for gemstone measurement. One ratti equals approximately 121.5 mg, roughly 0.6 carats. Some Indian jewelers and astrologers still reference ratti for traditional purposes, but the metric carat is the legal and commercial standard in India.