What is the difference between 22K and 24K gold?

24K gold is 99.9% pure (fineness 999). Too soft for everyday jewellery — used for coins and bars. 22K is 91.6% pure (fineness 916) with the remaining 8.4% being silver, copper, or zinc for hardness. Used for almost all wearable gold jewellery in India. 18K (750) is harder still, used for diamond-set or stone-set jewellery.