A coin is usually a piece of hard material, generally metal, usually in the shape of a disc, and most often issued by a government. Coins are used as a form of money in transactions of various kinds, from the everyday circulation of the United States quarter, to the storage of vast amounts of bullion. In the present day, coins and banknotes make up the cash forms of all modern money systems. Coins made for circulation (general monetized use) are usually used for lower-valued units, and banknotes for the higher values; also, in most money systems, the highest value coin is worth less than the lowest-value note. The face value of circulation coins is usually higher than the gross value of the metal used in making them.
Exceptions to these rules occur for non-monetized "bullion coins" made of silver or gold (and, rarely, other metals, such as platinum or palladium), intended for collectors or investors in precious metals. For examples of modern gold collector/investor coins, the United States mints the American Gold Eagle, Canada mints the Canadian Gold Maple Leaf, and South Africa mints the Krugerrand. The U.S. Eagle coin has a "face value" of ten U.S. dollars (and theoretically could be used to pay for goods or services worth ten dollars), and the Double Eagle has a "face value" of twenty U.S. dollars, but these denominations are historical references to pre-depression times when this was the case; of course, these face values are no longer relevant, since the market value of the coin, and the gold in it, is many times the face amount. The Canadian Gold Maple Leaf also has nominal face value, but the Krugerrand does not.
Historically, a great number of coinage metals (including alloys) and other materials have been used practically, impractically (i.e., rarely), artistically, and experimentally in the production of coins for circulation, collection, and metal investment, where bullion coins often serve as more convenient stores of assured metal quantity and purity than other bullion.