Because gold is an element, a lump of pure gold contains nothing but gold atoms. Because pure gold is very soft and easily marred, it is often alloyed with other metals to make jewelry, coins and other precious objects.
To indicate the fraction of gold contained in a particular piece, gold is stamped with a karat (K) number. Because pure gold is defined as 24K, an 18K gold necklace, for instance, is made of 18 parts pure gold and six parts other metals. Typical alloy choices include copper and silver, though platinum, aluminum and iron can be used to achieve particular colors.