A coin is a piece of hard material 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: these coins are usually worth less than banknotes: usually the highest value coin in circulation (i.e. excluding bullion coins) 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, or the general public may decide to melt the coins down or hoard them (see Gresham’s law). Exceptions to the rule of face value being higher than content value also occur for some bullion coins made of silver or gold (and, rarely, other metals, such as platinum or palladium), 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 (purely symbolic) face values; the Krugerrand does not. Historically, a great quantity of coinage metals (including alloys) and other materials (e.g. porcelain) 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.