Pure gold is 24 carat which is very malleable. But for the purpose of making ornaments or jewellery, it is generally hardened by adding the other metals like copper or gold which is an alloy. Thus, '18 carat' gold means an alloy of gold in which 18 parts by weight of pure gold is alloyed with 6 parts by weight of the other metals like Cu or the silver i.e. 18 carat gold contains 75% pure gold and rest is other metals like Cu of Ag etc. and its fitness is 750.

Carat : Gold content of an alloy of gold is expressed in carats. The word 'carat' signifies the fitness or purity of gold i.e. the number of parts by mass of gold in 24 parts by mass of the alloy.