Karat is a fraction. 24k is pure gold; 18k is eighteen parts gold in twenty-four, so 75% gold; 14k is 58.3%; 10k is 41.7%. The rest is copper, silver, nickel or zinc — metals that cost almost nothing.
So a 10k chain that weighs 30 grams contains about 12.5 grams of actual gold. An 18k chain weighing 20 grams contains 15 grams. The lighter chain is worth more, and it will keep being worth more no matter what the scale says.
The arithmetic is worth doing yourself before you come in: weight in grams, times the karat fraction, times the day’s spot price per gram. That gives you the pure gold content. Any honest buyer pays a percentage of that number, and any honest buyer will show you all three figures on a screen. If someone quotes you a single flat number for a bag of mixed gold without weighing the karats separately, they are counting on you not knowing this.
