Coins have been used as a form of currency across the world for almost three thousand years. Much like today, the earliest coins were formed from metals, with more precious metals reflecting higher monetary value. The most common shape of coin is a flat, rounded disc shape, which also tends to be small in size. Antique and rare coins can hold high value on the collectors' market, and some coins produced by the United States can be of particular interest to numismatists. Since 1792, coins of the US dollar have been minted and circulated throughout the States; the most common are pennies, nickels, dimes and quarters. Less common coins - but still minted frequently - are the half-dollar and one-dollar coins; originally, one-dollar coins were struck in gold. Between 1975-6, the United States produced a series of Bicentennial coins, commemorating 200 years since the Declaration of Independence.