Bi-metallic coins are coins consisting of more than one metal or alloy, generally arranged with an outer ring around a contrasting center. Common circulating examples include the €1, €2, British £2, Canadian $2, South African R5, Turkish 1 lira, IDR 1K, Hong Kong $10, Philippine P10.00 coin and all Mexican coins of $1 or higher denomination.
Bi-metallic coins have been issued for a long time, with examples known dating from the 17th century, while the Roman Empire issued special occasion, large medallions with a center of bronze or copper and an outer ring of orichalcum, starting with the reign of Hadrian. The silver-center cent pattern produced by the United States in 1792 is another example. In the 1830s and 1840s, British medalist Joseph Moore produced large numbers of bi-metallic "penny model" and less common "halfpenny model" tokens, as a proposal to replace the relatively large penny and halfpenny coins. Though not legal tender, Moore's tokens were circulated widely and accepted at face value by many merchants. Despite their popularity, the Royal Mint rejected the proposal, and did not reduce the size of the penny and halfpenny until decimalization.
In recent times, the first circulating bi-metallic coin was the Italian 500 lire, first issued in 1982. Morocco, with a 5 dirhams coin in 1987; France, with a 10 franc coin in 1992; Monaco, with a 10 franc; Thailand, with a 10 baht, in 1988; and Hong Kong, with a $10 coin, in 1993, issued bi-metallic coins for circulation all based on the Italian 500 lire's minting process. India introduced 10 rupee bi-metallic coins in 2009 that are dated 2006 (minted at Noida). Since 1996, Canada has also produced bi-metallic $2 coins (nicknamed "Toonies"), and Great Britain has issued a bi-metallic 2 pounds coin since 1997.
The first ever tri-metallic circulating coins were 20 franc coins introduced in France and Monaco in 1992. These were similar to the corresponding bi-metallic 10 franc coins, but had two rings instead of one. In 2015 the Reserve Bank of New Zealand introduced a circulating tri-metallic 50c coin.
As well as circulating coins, where they are generally restricted to high denomination coins, bi-metallic coins are often used in commemorative issues, often made of precious metals. For example, the only bi-metallic coin of the United States is the $10 Library of Congress commemorative, made of a gold ring around a platinum center. They are used primarily as a way of securing against coin counterfeiting.
The manufacturing process is similar to that of ordinary coins, except that two blanks (the inner and the outer) are struck at the same time, deforming the separate blanks sufficiently to hold them together.