Carat
The carat (symbol: ct) is a unit of mass equal to exactly 200 milligrams (0.2 grams), primarily used to measure the weight of gemstones, pearls, and sometimes diamonds.[1] This standardized metric derives its name and historical basis from the carob seed (Ceratonia siliqua), whose small, uniform seeds were employed in ancient trade—particularly in the Mediterranean region—as a natural weight reference for valuing precious stones, with one carat approximating the average weight of such a seed.[1] The modern definition of the carat was formally established in 1907 at the Fourth General Conference on Weights and Measures in Paris, where it was set at 200 milligrams to ensure international consistency in gemstone valuation and trade.[2] This metric carat was subsequently adopted by the United States in 1913 and the United Kingdom in 1914, replacing earlier variable standards like the British carat (approximately 205 milligrams) that had persisted since medieval times.[3] Prior to standardization, carat weights varied by region and commodity, often tied to local seed varieties or arbitrary divisions of larger units, leading to inconsistencies in global commerce.[4] Importantly, the carat should not be confused with the karat (or carat in some regions outside the U.S.), a separate measure denoting the purity of gold in an alloy, where 24 karats signify pure (99.9%) gold and lower values indicate proportions of alloying metals.[5] In jewelry and gemology, carat weight plays a central role as one of the "4Cs" (alongside cut, color, and clarity) in assessing a stone's quality and market value, with larger carats generally commanding higher prices due to their rarity, though size alone does not determine overall worth.[4] Today, the carat remains the universal standard for gemstone measurement, facilitating precise appraisals and transactions worldwide.[1]Definition and Modern Usage
Current Standard
The carat (symbol: ct) is a unit of mass equal to exactly 200 milligrams (0.2 grams or $2 \times 10^{-4} kg), serving as the international standard for measuring the mass of precious stones and pearls. This metric definition ensures uniformity in the global gem trade, where even minor weight differences significantly impact valuation.[6] The unit was formally defined at the 4th General Conference on Weights and Measures (CGPM) in 1907 to standardize commercial practices for gemstones, replacing varied historical systems. This adoption integrated the carat into the metric framework, and its value was reaffirmed in the 2019 revision of the International System of Units (SI), preserving its exact relation to the kilogram as a stable, non-SI unit accepted for specialized use.[6][7] For finer precision in jewelry, one carat is subdivided into 100 points, with each point equivalent to 2 milligrams; this allows gemologists to specify weights to the hundredth of a carat, such as a 0.25-carat stone denoted as 25 points. In gemological terminology, a "paragon" designates a flawless diamond of 100 carats or more, corresponding to 20 grams, representing an exceptional benchmark of size and perfection. The American National Standards Institute (ANSI) uses the abbreviation CD for carat in diamond-related contexts, as outlined in the ASC X12 EDI standards for electronic data interchange.[8][9][10]Applications in Gemology
In gemology, the carat serves as the primary unit for measuring the weight of diamonds, pearls, and other precious gemstones, enabling standardized evaluation and trade. This metric system, where one carat equals 200 milligrams, facilitates precise assessment of a stone's size and rarity, which are critical for determining its market value. Unlike size measurements that vary by shape, carat weight provides an objective metric that directly correlates with the volume of material in the gem, influencing its overall appearance when set in jewelry.[8] As one of the four key attributes known as the 4Cs—alongside cut, color, and clarity—carat weight plays a central role in diamond grading systems established by authoritative bodies like the Gemological Institute of America (GIA). Larger carat weights generally command exponentially higher prices due to the increasing rarity of stones above one carat, though this premium is moderated by the interplay of the other 3Cs; for instance, a high-clarity, well-cut diamond of 2 carats may far exceed the value of a lower-quality stone of the same weight. In practice, gemologists evaluate carat weight to contextualize a diamond's presence and desirability, with stones over 1 carat often highlighted for their rarity in retail and auction settings.[11][12][13] Certification processes in gemology rely on accurate carat measurements to verify authenticity and quality, with diamonds and loose gemstones typically weighed to the nearest 0.01 carat for retail descriptions, while laboratory reports like those from GIA extend precision to the fifth decimal place (0.00001 carat) for identification purposes. Fractional carats, such as 0.50 ct or 1.25 ct, are common in trade classifications, allowing buyers to compare stones systematically. For pearls, carat weight is applied similarly to loose specimens, though strung or mounted pearls may be weighed in grams; this ensures compliance with international standards in global trade. Modern electronic balances, often calibrated to 0.001 carat accuracy, are standard in gemological labs for these measurements, supporting efficient certification and valuation.[14][15][16] While the carat is indispensable for gemstones, it is not used for weighing precious metals like gold, which instead employs grams or troy ounces in trade to distinguish weight from purity assessments. This separation avoids confusion with the karat unit, which denotes gold fineness rather than mass.[17][18]Etymology and Origins
Linguistic Roots
The word "carat" traces its etymology to the Italian "carato," which derives from the Arabic "qīrāṭ," referring to the carob seed, ultimately originating from the Greek "kerátion," a diminutive of "kéras" meaning "horn," in allusion to the seed's horn-like shape.[19][5] This linguistic path reflects the term's evolution through Mediterranean languages, where the carob seed served as a natural reference for small weights in trade.[20] In English, "carat" first appeared in the mid-16th century as a measure of gold's fineness, with the earliest recorded use dating to 1552; by the 1570s, it had extended to denote the weight of diamonds and other precious stones.[21][19][20] This adoption aligned with growing European trade in gems and metals, adapting the term from earlier Romance language borrowings.[22] Culturally, the carat's nomenclature is rooted in the carob tree (Ceratonia siliqua), native to the Mediterranean region, whose small, uniform, horn-shaped seeds symbolized a reliable unit in ancient trade practices among Greek, Roman, and Arab merchants.[23][24] These seeds' consistent size made them a practical emblem for the weight standard, influencing the term's enduring association with precision in valuing precious materials.[25]Early Weighing Practices
The use of carob seeds (Ceratonia siliqua) as informal weights dates back to antiquity in the Mediterranean region, where jewelers employed these small, naturally occurring objects to measure small valuables such as gemstones and precious metals. Noted for their relative uniformity, carob seeds averaged approximately 0.2005 grams each (from female trees), making them a practical choice for balance scales in the absence of standardized metallic weights. This practice originated among ancient Greek and Roman traders, who valued the seeds' availability from the carob tree native to the area.[23] The carat system based on carob seeds spread along Arabic trade routes during the medieval period, particularly from the 8th century onward, facilitating commerce in pearls, spices, and jewels across the Islamic world. By the Renaissance, Italian merchants in city-states like Venice and Genoa adopted and propagated this method throughout Europe, integrating it into broader Mediterranean exchange networks. In some regions, such as parts of Northern Europe, barley grains were used as alternatives to carob seeds due to local availability, though they offered similar but less consistent uniformity for weighing fine goods. The term "carat" itself derives from the Greek keration, referring to the carob seed, underscoring its botanical roots.[25] Despite their convenience, carob seeds had significant limitations as weighing standards, with mass variations of up to 23% (coefficient of variation 22.7%) attributable to environmental factors like tree sex (female vs. hermaphrodite), pod position, and growing conditions, which introduced inconsistencies in trade and valuation. These discrepancies often led to disputes among merchants until more precise methods emerged. By the Renaissance era, the carat transitioned toward formalization in brass or metal scales, calibrated to seed equivalents for greater reliability while retaining the unit's foundational scale.[23]Historical Development
Pre-Standardization Variations
Before the international standardization of the carat in 1907, definitions of the unit varied significantly across regions and trade centers, leading to a range of weights from approximately 187 mg in Cyprus to 216 mg in Livorno (also known as Leghorn).[26] These pre-standardization carats were typically derived as fractions of local pound or ounce systems, reflecting the absence of a unified metric framework.[27] Key examples illustrate this diversity: the Venetian carat weighed 207 mg, while the Parisian carat was standardized locally at around 205.5 mg, though earlier French variations approached 188.8 mg based on apothecaries' measures tied to the marc du Paris.[27][28] In Amsterdam, the carat measured 205.1 mg, often linked to the Dutch troy pound. Such inconsistencies stemmed from reliance on national weight standards, including troy ounces for precious metals and gems in Britain and the Netherlands versus avoirdupois systems elsewhere, which complicated cross-border transactions and frequently sparked disputes in the international gem trade over valuation and authenticity.[27][4] Efforts to address these variations gained momentum in the 19th century, culminating in the 1875 International Congress in Paris, where delegates proposed a metric carat of 205 mg to harmonize gem weighing practices; however, this suggestion was not universally adopted and required further conferences for resolution.[29] These origins trace back briefly to ancient practices using carob seeds as counterweights, whose inconsistent sizes contributed to early disparities.[4]Standardization Process
The standardization of the carat reached a pivotal moment with its definition as the metric carat, exactly 200 milligrams, adopted at the 4th General Conference on Weights and Measures (CGPM) in Paris from 15 to 22 October 1907. This resolution aimed to unify commercial practices for diamonds, fine pearls, and precious stones amid varying regional definitions that had caused inconsistencies in international trade. In France, where the conference was hosted, the metric carat was promptly integrated into national law, with formal adoption on 22 June 1909 and compulsory use effective from 1 January 1912.[30][3] The metric carat's international dissemination accelerated in the ensuing decade, with ratification by numerous countries reflecting diplomatic efforts to harmonize gemology standards. Spain led early adoption via a Royal Order on 11 March 1908, followed by Italy, Bulgaria, Denmark, Norway, Japan, Portugal, Romania, Switzerland, Sweden, Germany, and the Netherlands. The United States implemented it via a Treasury Department order on 17 June 1913, effective 1 July 1913, while the United Kingdom authorized it under the Weights and Measures (Metric System) Act of 1897 and legalized it by Order in Council on 14 October 1913, taking effect on 18 April 1914. By the 1920s, the majority of trading nations had embraced the metric carat, facilitated by endorsements from bodies like the American Institute of Mining Engineers.[3][31] The metric carat's status was further solidified within the International System of Units (SI), established by the 11th CGPM in 1960 as an accepted non-SI unit for use alongside SI units, with oversight by the International Bureau of Weights and Measures (BIPM). This inclusion ensured its continued relevance in precise measurements, and the definition remained unchanged in the 2019 SI revision, which redefined base units but preserved accepted units like the carat. Adoption faced hurdles in the British Commonwealth, where imperial units lingered due to historical preferences, delaying full alignment until the mid-20th century when global trade imperatives drove universal implementation.[32]Specific Historical Systems
Regional and National Definitions
Before the international standardization of the metric carat at 200 mg in the early 20th century, carat weights for gemstones varied significantly across regions and nations, often derived from local troy or apothecary systems and calibrated against natural seeds like carob or gunja. These variations complicated global trade in precious stones, as merchants had to convert between local standards. In the United Kingdom, the Board of Trade established an official carat prior to 1888 equivalent to 205.409 mg, subdivided into four pearl grains of 51.25 mg each, though this subdivision was not applied to weighing pearls themselves, which used separate units like the pearl grain (approximately 51.84 mg based on troy equivalents). Following revisions in 1887, the UK carat was adjusted to 205.3035 mg to align more closely with international proposals, yet it remained distinct from the emerging metric system.[33][34] Other European nations maintained their own carat definitions during the 18th and 19th centuries. The Dutch carat, particularly in Amsterdam—a major gem trading center—weighed 205.522 mg, reflecting local commercial practices tied to older Dutch weight systems. In Russia, the carat was standardized at 205 mg, used consistently for diamonds and other gems in imperial trade contexts. These values hovered around 205 mg, a common approximation derived from four troy grains (each 64.79891 mg), but precise calibrations differed by locale to account for regional balance scales and reference artifacts.[33] In Asia, traditional units persisted alongside colonial influences. The Indian rupee carat, or ratti used in gemology, approximated 182 mg based on the weight of gunja seeds (Abrus precatorius), though variants like the pakki ratti reached up to 187 mg in some mercantile contexts before British standardization efforts in the late 19th century. This unit was integral to the subcontinent's jewelry trade, where gems were weighed for astrological and ornamental purposes. The Persian carat, pre-1933, equaled 191.67 mg (1/48 dirhem), influencing Middle Eastern gem markets.[33][35] The United States initially relied on troy-based equivalents, with the pre-1913 carat at 205.304 mg, varying slightly by manufacturer but generally aligned with British standards for import duties and certification. In 1913, the National Institute of Standards and Technology (then the Bureau of Standards) adopted the metric carat of 200 mg for official use in gem weighing and customs, marking a shift toward international uniformity and resolving discrepancies in transatlantic trade. This adoption was effective from July 1, 1913, and applied to diamonds and other precious stones.[33][31] The following table summarizes select historical carat weights from various regions, illustrating the pre-standardization diversity (all values in milligrams):| Region/Country | Carat Weight (mg) | Period/Context | Source |
|---|---|---|---|
| UK (Board of Trade, pre-1888) | 205.409 | Gem trade | Encyclopaedia of Scientific Units, Weights and Measures[33] |
| UK (Board of Trade, post-1887) | 205.304 | Revised standard | Encyclopaedia of Scientific Units, Weights and Measures[33] |
| Dutch (Amsterdam) | 205.522 | 18th–19th century trade | Encyclopaedia of Scientific Units, Weights and Measures[33] |
| Russian | 205.000 | Imperial gemology | Encyclopaedia of Scientific Units, Weights and Measures[33] |
| US (pre-1913) | 205.304 | Troy-based | Encyclopaedia of Scientific Units, Weights and Measures; NIST Circular 43[33][31] |
| Indian (ratti variant) | 182.000 | Traditional seed-based | Encyclopaedia of Scientific Units, Weights and Measures; gem trade references[33][35] |
| Persian (pre-1933) | 191.670 | Dirhem-derived | Encyclopaedia of Scientific Units, Weights and Measures[33] |
| French (obsolete, 1877 proposal) | 205.300 | Pre-metric gem use | NIST Circular 43[31] |
| German (pre-1907) | 205.300 | Regional variation | Encyclopaedia of Scientific Units, Weights and Measures[33] |
Greco-Roman and Refining Contexts
In ancient Greco-Roman contexts, the carat originated as a unit of mass tied to coinage standards, particularly the Roman siliqua, which represented one twenty-fourth of the gold solidus coin introduced by Emperor Constantine around 312 AD.[19] The siliqua, equivalent to the Greek keration (κεράτιον), derived its name from the carob seed (Ceratonia siliqua) and weighed approximately 189 milligrams in its theoretical form, serving as a fundamental measure for weighing gold and other precious materials.[3] This unit equated to four Roman grains (granum), reflecting a subdivision where the keration provided a consistent, seed-based reference for small-scale precious metal transactions in trade and minting. During the medieval period in England, the carat adapted for gold refining and assaying under the troy weight system, where the "refiners' carat" or "pound carat" measured 240 troy grains, comprising one twenty-fourth of the troy pound used exclusively for precious metals. For finer alloy assessments, the "ounce carat" was defined as 20 troy grains, allowing precise evaluation of gold-silver mixtures in bullion and jewelry production.[36] This system subdivided each carat into four quarters (grains), enabling assayers to quantify impurities and fineness in gold alloys through weight-based assays, a practice standardized by the 15th century in English mints and refineries. The Greco-Roman carat's influence extended to gold purity standards, establishing the 24-carat system where a pure gold solidus—weighing the equivalent of 24 siliquae—defined 24-carat gold as unalloyed, with lower caratage indicating proportional alloy content for durability in coinage and artifacts.[19] However, these historical masses, rooted in the variable carob seed or theoretical siliqua, differed from the later gemological carat, which evolved into a more uniform metric for stones rather than metals.[3] Historical ambiguities in the carat's mass arose from the siliqua's evolution in Roman coinage, where the gold solidus standard of approximately 4.5 grams (yielding 189 mg per siliqua) stabilized post-Constantine but saw silver siliquae weights decline to around 1.5 grams or less by the early 5th century, including around 417 AD under Honorius, reflecting debasement amid empire-wide economic pressures.[37] This progression clarified the unit's role beyond seeds, emphasizing its fixed relation to the solidus for gold rather than fluctuating coin weights.[19]Distinctions and Related Units
Difference from Karat
The carat is a unit of mass used primarily for weighing gemstones, equivalent to 200 milligrams, whereas the karat (often abbreviated as K or kt) is a measure of the purity of gold in an alloy, expressed as parts of pure gold out of 24 total parts. For instance, 24-karat gold consists of 99.9% pure gold with no alloying metals, while 18-karat gold contains 75% pure gold (18/24 parts) mixed with other metals such as copper or silver for added durability.[38][5][39] Both terms trace their origins to the Greek word kerátion, meaning "carob seed," which was used in ancient times as a consistent small-weight reference due to the uniform size of carob seeds; this evolved through Arabic qīrāṭ and into Medieval Latin and Old French forms. Over time, the terms diverged in usage: "carat" retained its association with mass measurement for gems and precious materials, while "karat" specifically denotes gold fineness, a distinction that became formalized in the 19th century. In American English, "karat" is the standard spelling for gold purity to avoid confusion with the mass unit, whereas internationally, "carat" is often used for both concepts.[5][19] A frequent source of confusion arises in jewelry contexts, where consumers may mistakenly equate a gemstone's carat weight—such as a 1-carat diamond—with the karat purity of accompanying gold settings, leading to misinterpretations of value or quality. Regulatory bodies address this through strict labeling guidelines; for example, the U.S. Federal Trade Commission (FTC) mandates that karat fineness for gold must be disclosed accurately and conspicuously alongside any "gold" claim, while gemstone weights in carats require precise measurement to the nearest decimal or range without exaggeration, ensuring no deceptive overlap in terminology.[38][40] There is no functional overlap between the two: the carat has never been applied to measure metal purity, and the karat does not denote any mass or weight. This separation prevents the karat from being misconstrued as a unit of measurement in gemology or other weighing applications.[5][41]Comparisons to Other Mass Units
The international metric carat is defined as exactly 200 milligrams (mg), equivalent to 0.2 grams (g).[42] Thus, five carats equal one gram, providing a straightforward relation to the metric system for gemstone measurements.[1] In imperial units, one carat equals approximately 3.086 troy grains, a subdivision historically used in precious metal weighing.[3] It also corresponds to about 0.00705 avoirdupois ounces, the standard ounce in everyday imperial mass systems.[43] For context in jewelry and precious metals, the troy ounce—commonly applied to bullion and containing 480 troy grains—weighs approximately 31.1 grams, or 155.5 carats.[44] Historically, the pearl grain served as a smaller unit in gem evaluation, defined as one-quarter carat or 50 mg.[34] To aid non-experts, one carat approximates the mass of a standard paperclip, emphasizing its small scale in daily terms.[42]| Unit | Equivalent to 1 Carat |
|---|---|
| Milligrams (mg) | 200 mg[42] |
| Grams (g) | 0.2 g[1] |
| Troy grains | ≈3.086 grains[3] |
| Avoirdupois ounces | ≈0.00705 oz[43] |
| Troy ounces (relation) | 1/155.5 troy oz[44] |
| Pearl grains | 4 pearl grains[34] |