Fifteen-carat gold is a gold alloy containing 62.5 per cent pure gold by mass — that is, 15 parts gold to 9 parts base metal, typically copper, silver, or a combination of both. It occupies a position between the more familiar 14-carat (58.3%) and 18-carat (75%) standards and is today recognised exclusively as a marker of British or British-influenced jewellery made before 1932. Pieces bearing the hallmark 15ct or the millesimal fineness stamp 625 are, by definition, antique or estate items, and their presence in a piece is itself a dating tool of considerable reliability.

Historical Context

The 15-carat standard was formally recognised under British assay law during the nineteenth century, when the jewellery trade required an alloy that offered greater economy than 18-carat gold without descending to the relatively low fineness of 9-carat. Victorian and Edwardian jewellers working in the naturalistic, archaeological-revival, and later Arts and Crafts idioms found 15-carat gold well suited to their purposes: it was workable, acceptably durable for the elaborate settings and fine wirework fashionable in the period, and carried a warm, slightly deeper colour than 18-carat yellow gold owing to its higher copper content in many formulations.

The standard coexisted with 12-carat gold (50% pure), another intermediate fineness that was likewise discontinued in 1932. Together, the two caratages were swept away when the United Kingdom rationalised its legal gold standards to four: 9-carat, 14-carat, 18-carat, and 22-carat. From 1 January 1932, no new 15-carat or 12-carat articles could be submitted for hallmarking at the British assay offices.

Identification and Hallmarking

British hallmarking practice requires that gold articles bear a carat mark alongside the assay office mark (anchor for Birmingham, leopard's head for London, castle for Edinburgh, and so forth) and, on pieces made after 1890, a date letter. A piece stamped 15ct or 625 will therefore typically also carry an assay office symbol and a date letter consistent with the period 1854–1931, providing a useful cross-check for the specialist or collector. The millesimal fineness mark 625 was introduced as an alternative notation in the late nineteenth century to facilitate trade with Continental European markets, where fineness numbers rather than carat fractions were standard.

Pieces imported into Britain from Continental Europe during the same period may occasionally carry a 15-carat equivalent fineness, though the precise alloy compositions and marking conventions varied by country. Caution is warranted when assessing unmarked or partially marked pieces: independent testing by X-ray fluorescence (XRF) spectrometry at a reputable assay office or gemmological laboratory is the definitive method for confirming alloy composition.

In the Antique and Estate Trade

For dealers, auctioneers, and collectors specialising in Victorian and Edwardian jewellery, the 15-carat mark is a routine and expected feature. It appears with particular frequency on mourning jewellery, seed-pearl and half-pearl work, turquoise and coral pieces, and the fine chain-link necklaces and bracelets characteristic of the late nineteenth century. The warm yellow tone of many 15-carat alloys is considered sympathetic to the gemstones and enamel work of the period.

From a valuation standpoint, the gold content of a 15-carat piece is calculated at 62.5 per cent of the total metal weight — lower than 18-carat but meaningfully higher than 9-carat. However, in the antique market, the intrinsic gold value is rarely the primary driver of price; maker, condition, design quality, and provenance typically dominate. A fine piece of Castellani or Carlo Giuliano work in 15-carat gold commands a premium based on its artistic and historical significance rather than its melt value.

Practical Considerations for the Jeweller

Jewellers undertaking repair or restoration work on 15-carat pieces face a minor but real challenge: no modern solder is specifically formulated for 625 fineness. In practice, 14-carat or 18-carat solders are used, with the choice guided by colour-matching and the structural requirements of the repair. Any addition of new metal will alter the overall fineness of the piece, a consideration relevant if the item is subsequently tested or re-assayed. Clients and conservators should be advised accordingly, and any significant restoration documented.