It could indeed be marked 18ct and still qualify as being an Antique, gold ring. In the States, in 1906, The national Gold & Silver Stamping Act of 1906 mandated that any item stamped with 18k or 18kt must reflect the accurate purity of the metal. In 1478 in the UK the standard was formalised and 18k, became the minimum legal purity standard for gold.
