Gold jewelry is rarely pure. Karat measures purity in 24ths: 24k is 99.9% gold, 18k is 75%, 14k is 58.3%, 10k is 41.7%. The per-gram value scales directly with that fraction.
To value a piece, take the live 24k spot price per gram, multiply by the karat fraction, then by the weight in grams. A 14k chain weighing 10g is worth 10 × spot × 0.583 in pure-gold terms.
Buyers pay a margin below melt value, so the melt figure is your ceiling, not the offer. Knowing it stops you accepting far below what the metal is worth.
Try it: GoldGramPrice on value gold per gram.