Sterling silver is an alloy of silver containing 92.5% by weight of silver and 7.5% by weight of other metals, usually copper. The sterling silver standard has a minimum millesimal fineness of 925.
Fine silver, for example 99.9% pure silver, is relatively soft, so silver is usually alloyed with copper to increase its hardness and strength. Sterling silver is prone to tarnishing, and elements other than copper can be used in alloys to reduce tarnishing, as well as casting porosity and firescale. Such elements include germanium, zinc, platinum, silicon, and boron. Recent examples of alloys using these metals include argentium, sterlium, sterilite and silvadium.
One of the earliest attestations of the term is in Old French form esterlin, in a charter of the abbey of Les Préaux, dating to either 1085 or 1104. The English chronicler Orderic Vitalis (1075 – c. 1142) uses the Latin forms libræ sterilensium and libræ sterilensis monetæ. The word in origin refers to the newly introduced Norman silver penny.
According to the Oxford English Dictionary, the most plausible etymology is derivation from a late Old English steorling (with (or like) a "little star"), as some early Norman pennies were imprinted with a small star. There are a number of obsolete hypotheses. One suggests a connection with starling, because four birds (in fact martlets) were depicted on a penny of Edward I.
The sterling alloy originated in continental Europe and was being used for commerce as early as the 12th century in the area that is now northern Germany.
In England the composition of sterling silver was subject to official assay at some date before 1158, during the reign of Henry II, but its purity was probably regulated from centuries earlier, in Saxon times. A piece of sterling silver dating from Henry II's reign was used as a standard in the Trial of the Pyx until it was deposited at the Royal Mint in 1843. It bears the royal stamp ENRI. REX ("King Henry") but this was added later, in the reign of Henry III. The first legal definition of sterling silver appeared in 1275, when a statute of Edward I specified that 12 Troy ounces of silver for coinage should contain 11 ounces 2 1⁄4 pennyweights of silver and 17 3⁄4 pennyweights of alloy, with 20 pennyweights to the Troy ounce. This is (not precisely) equivalent to a millesimal fineness of 926.
In Colonial America, sterling silver was used for currency and general goods as well. Between 1634 and 1776, some 500 silversmiths created items in the “New World” ranging from simple buckles to ornate Rococo coffee pots. Although silversmiths of this era were typically familiar with all precious metals, they primarily worked in sterling silver. The colonies lacked an assay office during this time (the first would be established in 1814), so American silversmiths adhered to the standard set by the London Goldsmiths Company: sterling silver consisted of 91.5–92.5% by weight silver and 8.5–7.5 wt% copper. Stamping each of their pieces with their personal maker's mark, colonial silversmiths relied upon their own status to guarantee the quality and composition of their products.