Carat

The purity of gold is described by its ‘fineness’ (parts per thousand) or by the carat (karat in the United States) scale. The word comes from the Greek karation, the Italian carato and the Arabic qirat, all meaning ‘fruit of the carob tree’. The carob seed was formerly used to balance the scales in Oriental bazaars. Because pure gold is soft and liable to wear down, it has always been alloyed with other metals to make it harder. The proportion of gold is defined by the carat scale. Pure gold is 24 carat (or 1,000 fine).

The proportion in jewellery varies considerably from country to country and is preserved not only by custom but often by law. The advantage of lower caratage is that a wider range of colour can be attained, from green to red and white golds, depending on the balance of other metals with which it is alloyed. Strength, hardness and hence wear and scratch resistance tend to increase as caratage is lowered. The validity of the caratage stamped on each piece of jewellery is often guaranteed by an official hallmarking system.

An entirely different carat scale is used for the measurement of the weight of diamonds. See also Gold’s Credentials.