Definition
A unit of measurement with two distinct legal applications:
1. Weight of precious stones. A carat is a unit of mass used to express the weight of diamonds and other precious stones. Historical legal dictionaries fix the carat at four grains (Black's 1st Ed., Bouvier) or three and one-sixth grains Troy (Black's 2nd Ed., Wharton). The modern international standard, established by convention in the twentieth century, is the metric carat of exactly 200 milligrams — a figure that supersedes all earlier grain-based equivalents.
2. Fineness of gold. Carat (also spelled karat, particularly in American usage) denotes the proportion of pure gold in an alloy, expressed on a scale of 24. Pure gold is 24 carats. An alloy described as 18 carats contains 18 parts gold and 6 parts other metals. This usage is independent of weight; it is a ratio, not a mass measurement.
Common Language
Modern common usage (Wiktionary): A metric unit of mass equal to exactly 200 mg, chiefly used for measuring precious stones and pearls; also a 24-point scale used to measure the purity of gold.
Historical common usage (Webster's 1913): The weight by which precious stones and pearls are weighed; a twenty-fourth part used in estimating the proportionate fineness of gold.
Editorial note: Common usage and legal usage largely overlap here, but researchers encounter two specific traps. First, historical legal sources record the carat at materially different grain weights than the modern metric standard — a gap that matters when construing older contracts, insurance policies, or appraisals. Second, the spelling divergence between "carat" (stones and gold fineness in British/international usage) and "karat" (gold fineness in American commercial usage) does not appear in historical legal dictionaries at all, meaning documents drafted before the twentieth century offer no spelling signal to distinguish the two applications.
Common Confusion
Carat (weight/fineness) is frequently confused with karat, which in modern American commercial and regulatory usage is the preferred spelling exclusively for gold fineness. The two spellings are functionally interchangeable in older legal documents and in British-influenced sources. Researchers should not treat the spelling distinction as legally operative in historical texts. A separate and unrelated confusion exists between carat and caret (a proofreading mark) and carrot — neither of which has legal significance but which produce retrieval noise in full-text corpus searches.
Recognized Forms
/SUBTYPES
Metric carat (ct): The international standard of 200 milligrams, adopted for gemstone weight. Governs modern trade, customs valuation, and consumer protection regulation.
Karat (kt or k): American commercial spelling denoting gold fineness on the 24-point scale. Encountered in Federal Trade Commission guidelines on jewelry marketing and in state consumer fraud statutes addressing misrepresentation of gold content.
Diamond grain: One-quarter of a carat in the historical gem trade, a subdivision noted in Black's 2nd Ed. Not a standard unit in modern commerce.
Why It Matters in Research
Researchers working with historical contracts, estate inventories, insurance policies, or customs records involving gemstones or gold will encounter carat weights expressed in grains — and the grain equivalent varies by source. Black's 1st Ed. and Bouvier agree on four grains; Black's 2nd Ed. and Wharton record three and one-sixth grains Troy. These are not the same number, and neither corresponds to the modern metric carat of 200 milligrams (approximately 3.086 grains Troy). A document from 1880 describing a diamond as weighing "two carats" cannot be converted to modern metric weight without knowing which grain standard was in use at the time and place of drafting.
For gold fineness, the carat/karat spelling split is a twentieth-century American development. Pre-1900 legal documents use "carat" for both applications without differentiation. Post-1950 American regulatory materials, FTC guidelines, and commercial statutes tend to use "karat" for gold and "carat" for stones. Running corpus searches on only one spelling will produce incomplete results when crossing that temporal or jurisdictional line.
Customs and tariff law is a particular area of sensitivity. Valuation of gems and gold for import duty purposes has historically turned on weight and fineness certifications, and disputes over measurement standards appear in administrative records and early customs cases. Researchers should cross-reference applicable weights-and-measures statutes contemporaneous with the document under examination.
Historical Dictionary Support
The three shelf sources are in partial agreement and partial conflict. Black's 1st Ed. and Bouvier agree that a carat equals four grains and is used for precious stones — both cite Webster as authority, suggesting they are drawing from the same lexicographic source rather than independent legal analysis. Black's 2nd Ed. is more precise and more useful: it records the three and one-sixth grains Troy figure (attributing it to Wharton), introduces the subdivision into diamond grains, and — crucially — extends the definition to cover gold fineness on the 24-carat scale. The 1st Ed. and Bouvier omit the gold fineness application entirely.
None of the historical legal dictionaries address the metric carat or the carat/karat spelling convention. This is an expected gap given their publication dates, but it means they provide no guidance for reading twentieth-century and later materials. For modern applications, researchers must look beyond the shelf dictionaries to weights-and-measures legislation, international treaty instruments, and regulatory definitions.
Jurisdictional Note
In the United States, the Federal Trade Commission's Guides for the Jewelry, Precious Metals, and Pewter Industries govern use of carat and karat terminology in commercial contexts, standardizing "karat" for gold fineness and "carat" for gem weight. In the United Kingdom and most Commonwealth jurisdictions, "carat" serves both functions. The metric carat of 200 milligrams is the international standard recognized across major trading jurisdictions, but older domestic statutes in some jurisdictions may still reference pre-metric grain-based definitions.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia: Weights and Measures — for the legal history of measurement standards and their role in contract construction and customs valuation.
See Law Mind Encyclopedia: Gemstones and Jewelry Law — for trade regulation, consumer protection, and appraisal standards involving carat weight.
See Law Mind Encyclopedia: Gold and Precious Metals Regulation — for karat/fineness standards in commercial law and FTC regulatory context.