. 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 often have images, numerals, or text on them. Coins are usually metal or an alloy, or sometimes made of manmade 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. Metal ingots, silver bullion or unmarked bars were probably in use for exchange among many of the civilizations that mastered metallurgy. The weight and purity of bullion would be the key determinant of value. In the Achaemenid Empire in the early 6th century BC, coinage was yet unknown, and barter and to some extent silver bullion was used instead for trade. The practice of using silver bars for currency also seems to have been current in Central Asia from the 6th century BC. 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. The earliest coins are mostly associated with Iron Age Anatolia of the late 7th century BC, and 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. The unpredictability of the composition of naturally occurring electrum implied that it had a variable value, which greatly hampered its development. 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, site of the earliest known deposit of electrum coins. Because the oldest lion head coins were discovered in that temple, and they do not appear to have been used in commerce, these objects may not have been coins but badges or medals issued by the priests of that temple. Anatolian Artemis was the ὀ ῶ, whose symbol was the stag. It took some time before ancient coins were used for commerce and trade. Even the smallest-denomination electrum coins, perhaps worth about a day's subsistence, would have been too valuable for buying a loaf of bread. Maybe the first coins to be used for retailing on a large-scale basis were likely small silver fractions, Hemiobol, Ancient Greek coinage minted by the Ionian Greeks in the late sixth century BC.