Karat mark tolerance is the permissible deviation between the karat fineness stamped on a gold article and the actual gold content as determined by assay. The concept is foundational to gold-jewellery regulation worldwide because no commercial alloy can be produced and worked to perfect mathematical precision, and the regulatory question is how much variance the trade will tolerate before a piece is considered misstamped.

The United States regime

The United States National Gold and Silver Stamping Act of 1906, as amended, and the Federal Trade Commission's Jewelry Guides at 16 CFR Part 23 govern karat-mark tolerance for goods sold in interstate commerce. The principal rule for unsoldered gold articles is that the actual fineness must not fall below the marked karat fineness by more than 3 parts per 1,000 — that is, a piece marked 14K (583 parts per 1,000) must assay at no less than 580. For soldered articles, where the solder alloy by necessity has a slightly different composition, the tolerance widens to 7 parts per 1,000, and a piece marked 14K with solder must assay at no less than 576. Articles that do not meet these tolerances are considered misstamped under federal law.

Plumb karat

The phrase "karat plumb" or "plumb gold" refers to articles that meet or exceed the stamped fineness without any tolerance, that is, a piece marked 14K plumb assays at 583 or higher. The plumb designation became commercially important in the United States in the late 1970s and 1980s as some manufacturers had relied on the older "karat with tolerance" practice — under which a 14K piece might assay at as low as 13.5K and still be considered legally compliant under earlier industry conventions. The 1976 amendment to the Stamping Act and the FTC's subsequent guides effectively abolished the older practice and required that stamped fineness be plumb, that is, the marked karat must be the minimum actual fineness within the narrow tolerances above.

International comparisons

The Indian BIS hallmarking regime works in millesimal fineness rather than karat, with similar logic: a 22K (916) hallmark requires actual fineness of at least 916 parts per 1,000, with no negative tolerance permitted under BIS Hallmarking. The United Kingdom Hallmarking Act 1973 specifies fineness standards for 9, 14, 18 and 22 carat gold and similarly does not permit negative tolerance below the marked fineness; the Assay Offices test each batch and mark only pieces meeting standard. The European Convention on the Control and Marking of Articles of Precious Metals (Hallmarking Convention, the Vienna Convention of 1972) likewise specifies plumb fineness with effectively zero negative tolerance.

Disclosure and enforcement

The trade implication is that any gold article sold into a regulated jewellery market should assay at or above its marked karat fineness within the narrow technical tolerances allowed for solder presence. Falling short of the marked fineness is a misstamping offence subject to FTC enforcement in the United States and to criminal sanctions under the various national hallmarking acts elsewhere. The shift to compulsory hallmarking in major markets — India in 2021-2022, ongoing tightening in the EU and Middle East — has made the karat-mark tolerance question one of compliance rather than industry custom.