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 China can be of particular interest to numismatists. Ancient Chinese coins were some of the earliest forms of this currency, taking the shape of spades or knives rather than the common round design. From around the mid 4th century BC, rounded coins with square or circular holes in the center came into circulation; these continued to be used until the fall of the Chinese Empire in the 20th century.