Coin. A coin is a small, flat, round piece of metal or plastic used primarily as a medium of exchange or legal tender. They are standardized in weight, and produced in large quantities at a mint in order to facilitate trade. They are most often issued by a government. Coins are usually metal or alloy, or sometimes made of synthetic materials. They are usually disc shaped. Coins made of valuable metal are stored in large quantities as bullion coins. Other coins are used as money in everyday transactions, circulating alongside banknotes. Usually the highest value coin in circulation is worth less than the lowest-value note. In the last hundred years, the face value of circulation coins has occasionally been lower than the value of the metal they contain, for example due to inflation. If the difference becomes significant, the issuing authority may decide to withdraw these coins from circulation, possibly issuing new equivalents with a different composition, or the public may decide to melt the coins down or hoard them. Exceptions to the rule of face value being higher than content value also occur for some bullion coins made of copper, silver, or gold, intended for collectors or investors in precious metals. Examples of modern gold collector/investor coins include the British sovereign minted by the United Kingdom, the American Gold Eagle minted by the United States, the Canadian Gold Maple Leaf minted by Canada, and the Krugerrand, minted by South Africa. While the Eagle, Maple Leaf, and Sovereign coins have nominal face values; the Krugerrand does not. Historically, a great quantity of coinage metals and other materials have been used to produce coins for circulation, collection, and metal investment: bullion coins often serve as more convenient stores of assured metal quantity and purity than other bullion. The first coins were developed independently in Iron Age Anatolia and Archaic Greece, India and China around the 7th and 6th centuries BCE. Coins spread rapidly in the 6th and 5th centuries BCE, throughout Greece and Persia, and further to the Balkans. Standardized Roman currency was used throughout the Roman Empire. Important Roman gold and silver coins were continued into the Middle Ages. Ancient and early medieval coins in theory had the value of their metal content, although there have been many instances throughout history of the metal content of coins being debased, so that the inferior coins were worth less in metal than their face value. Fiat money first arose in medieval China, with the jiaozi paper money. Early paper money was introduced in Europe in the later Middle Ages, but some coins continued to have the value of the gold or silver they contained throughout the Early Modern period. The penny was minted as a silver coin until the 17th century. The first copper pennies were minted in the United States in the 1790s. Silver content was reduced in many coins in the 19th century, and the first coins made entirely of base metal, representing values higher than the value of their metal, were minted in the mid 19th century. Coins were an evolution of currency systems of the Late Bronze Age, where standard-sized ingots, and tokens such as knife money, were used to store and transfer value. In the late Chinese Bronze Age, standardized cast tokens were made, such as those discovered in a tomb near Anyang. These were replicas in bronze of earlier Chinese currency, cowrie shells, so they were named Bronze Shell. Further information: Archaic period of ancient Greek coinage According to Aristotle and Pollux, the first issuer of coins was Hermodike of Kyme The earliest coins are mostly associated with Iron Age Anatolia, especially with the kingdom of Lydia. Early electrum coins were not standardized in weight, and in their earliest stage may have been ritual objects, such as badges or medals, issued by priests. Many early Lydian and Greek coins were minted under the authority of private individuals and are thus more akin to tokens or badges than to modern coins, though due to their numbers it is evident that some were official state issues, with King Alyattes of Lydia being a frequently mentioned originator of coinage. The first Lydian coins were made of electrum, a naturally occurring alloy of silver and gold that was further alloyed with added silver and copper. Most of the early Lydian coins include no writing, only an image of a symbolic animal. Therefore, the dating of these coins relies primarily on archaeological evidence, with the most commonly cited evidence coming from excavations at the Temple of Artemis at Ephesus, also called the Ephesian Artemision.