A bullion coin is an investment-grade coin that is valued by its weight and fineness of a specific precious metal. Unlike commemorative or numismatic coins valued by limited mintage, rarity, condition, and age, bullion coins are purchased by investors seeking a simple and tangible means to own and invest in the gold, silver, platinum, and palladium markets. The United Kingdom defines investment coins more specifically as coins that have been minted after 1800, have a purity of not less than 900 thousandths and are, or have been, legal tender in their country of origin. Under United States law, "coins" that fail the last of these requirements are not coins at all, and must be advertised as "rounds" instead. The American Eagle and Canadian Gold Maple Leaf series are the only coins available in gold, silver, platinum, and palladium.