Why does the karat matter when you choose a gold ring? Pure gold is too soft for engagement rings, so it is mixed with other metals to make an alloy (a blend of metals) 2. The karat tells you how much of that alloy is gold. One karat is one part out of 24, so pure gold is 24 karat 2.
Why does 18 carat gold have other metals in it? 18 carat gold contains 75 per cent gold and 25 per cent other metals. These added metals make gold more durable and can change its colour: copper gives rose gold, while palladium or silver give white gold 3. Counted in parts, 18K gold is 18 parts gold and 6 parts other metals, such as copper, silver or zinc 1.
Why can the same word mean two different things? Karat is also spelled "carat", but it is not the same as the carat used to weigh gems 4. A gem carat is a unit of weight equal to 0.20 gram, shortened to ct 1. So "18ct gold" describes purity, while a "1.00 ct diamond" describes how heavy a stone is. Rings may also show fineness, the gold content in parts per thousand, so an 18K ring might be stamped 750 or 18K 2. Read more in What 375, 585 and 750 Gold Marks Mean and 9ct vs 14ct vs 18ct Gold.
Example: You look inside a ring and see the number 750. Fineness counts gold in parts per thousand, so 750 parts out of every 1,000 are gold. That is the same as 18 parts out of 24: the ring is 18ct (18K) gold, and the other quarter is alloying metals 2.