2 main ways are used to measure the purity when you buy gold coins: Karat and Fineness. 24 Karat (KT) is considered to be purest form of gold which contains 24/24 parts of gold. Similarly, 22KT gold contains 22 parts of gold and 2 parts of other metals such as zinc or silver, which is used to make long-term products such as jewelry
BIS
While buying gold coins, even ‘pure’ gold can contain minute amounts of mixed impurities which the manufacturer has not been able to remove. Fineness is defined as weight of precious metal (such as gold) in proportion to the total weight (which includes alloy and impurities). It is expressed in units of parts per 1,000. The purest form of gold, i.e., 24KT, when measured in fineness shows 999.9 parts per thousand of purity.
BIS
Usually, gold coins are available in the market ranging from denominations of 0.5 grams to 50 grams in weight. However, availability of the denomination you wish to buy depends on the jeweler.