Pure gold is 24 carats. It is very soft so, it cannot be used for making jewellery. Hence, it is alloyed with silver or copper to make it hard to make jewellery. A fineness of 100% is impossible to achieve as there will be some degree of impurity in commercially available gold. It indicates that there is no other metal mixed with the gold. As the cartage of gold reduces, it indicates the existence of other metals, typically copper and silver, mixed with gold. For example, 18-carat gold has 75% (18/24) of gold and 25% of other metals. If some copper is replaced with a silver (keeping the gold content constant), the alloy takes on a yellow-green hue or a rose colour, depending on the mixture of copper and silver.
Another way of denoting the purity of gold is by the fineness. Fineness represents the pure gold in the gold alloy in parts per thousand. A fineness of 99.5 or .995 indicates 995 parts of gold and five parts of another metal.