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The karat system is used to reveal the amount of pure gold found in an item.
24 karat (24K gold) is pure gold.
18 karat (18K gold) contains 18 parts gold and 6 parts another metal or metals, making it 75% gold.
14 karat (14K gold) contains 14 parts gold and 10 parts another metal or metals, making it 58.3% gold.
10 karat (10K gold) contains 10 parts gold and 14 parts another metal or metals, making it 41.7% gold. 10k gold is the minimum karat designation that can still be called gold in the US.
Sterling silver is an alloy of silver containing 92.5% pure silver and 7.5% other metals, usually copper. The sterling silver standard has a minimum millesimal fineness of 925.