Hallmarks stamp purity as a karat number (14K) or a millesimal figure (585 = 58.5%). A genuine stamp is a strong signal, but counterfeits exist, so value rests on actual purity, not just the mark.
Jewelers verify with acid tests, electronic conductivity testers, or XRF analyzers; XRF is non-destructive and reads exact composition.
Once you know the true karat and weight, the melt value is live 24k price per gram × purity fraction × grams — the floor any honest buyer works from.