The karat stamp is the small numeric mark, ordinarily impressed by tool or laser-engraved on a discreet location of a gold jewellery piece, indicating the karat fineness of the gold alloy. Common stamps include 24K (or 999), 22K (916), 18K (750), 14K (585), 10K (417) and 9K (375). The stamp is a regulatory disclosure, a consumer-protection mechanism, and the principal way that finished jewellery carries forward the fineness information of its raw alloy.

Placement

Conventional placement is on a flat or near-flat interior or hidden surface — the inside of a ring shank, the underside of a clasp, the back of a brooch or pendant, the inside of an earring post or back. The stamp is positioned where it will not interfere with the design or with sizing operations, and where it remains legible after wear. Manufacturer's marks (maker's mark, sponsor's mark) typically appear adjacent to the karat stamp, and in regulated hallmarking jurisdictions the assay-office mark and date letter join them.

Stamp formats

The North American convention is to mark with the karat number followed by K, sometimes with the suffix P (plumb): 14K, 18K, 14KP. The European convention is millesimal fineness without suffix: 750 for 18-karat, 585 for 14-karat, 375 for 9-karat. Some pieces in the bridge between markets, particularly export goods, carry both. The Indian BIS hallmark uses the millesimal fineness number plus the BIS logo, the assay-centre identifier, the year letter and the jeweller's identifier. The United Kingdom Assay Office hallmark stack includes the millesimal fineness, the assay-office mark (London leopard's head, Birmingham anchor, Sheffield rose, Edinburgh castle), the date letter and the sponsor's mark.

Regulatory framework

The legal status of the karat stamp varies by jurisdiction. In the United States the National Gold and Silver Stamping Act and the FTC Jewelry Guides regulate the relationship between stamped and actual fineness but do not require independent third-party assay; the manufacturer's mark accompanying the karat stamp identifies who is legally responsible for accuracy. In the United Kingdom and the Hallmarking Convention member states, third-party assay-office testing is mandatory before pieces above a small minimum weight may be sold, and the karat stamp without an accompanying official hallmark is not lawful. India's BIS hallmarking became compulsory across most categories in 2021-2022. The result is that an apparently identical karat stamp on a piece sold in different jurisdictions may carry very different evidentiary weight depending on whether independent assay sits behind it.

Trade significance

For trade buyers the karat stamp is necessary but not sufficient evidence of fineness. Independent assay (touchstone, XRF, fire assay) is the standard verification practice for high-value items and for parcels of unknown provenance, and the relationship between the stamped value and the actual fineness is the routine basis for trade disputes when discrepancies emerge.