An extremely rare "rainbow cup" coin minted more than 2,000 years ago by the Celts has been found next to a river in Germany, according to the Bavarian State Archaeological Collection.

The gold coin, minted in the second or first century B.C., features a rare design of a four-pointed star surrounded by arches on one side, said Bernward Ziegaus, a senior curator in the State Archaeological Collection's numismatic department who is studying the coin. Like other rainbow cups, the coin is curved.

"The name rainbow cup coins come from the legend that they are drops of gold that fall to earth at the end of a rainbow," Ziegaus told Live Science in an email. "Another legend about these Celtic coins tells us that these coins can only be found by Sunday children," or a child of fortune.

"In fact, the finder was born on a Sunday and is indeed a Sunday child, a lucky child!" he said.

The finder, a collaborator with state archaeological officials, discovered the coin this spring about 45 miles (70 kilometers) west of Munich on the Lech River in the southern state of Bavaria.

The "heads" side of the 0.5-inch-wide (13 millimeters) coin "shows a stylized human head with a large eye," with the nose and lips depicted as dots, Ziegaus said. A metal analysis revealed that the coin is 77% gold, 18% silver and 5% copper.

There are only three known rainbow cups with the star-and-arch motif. "The interpretation of the motive is difficult," Ziegaus said. "The star is perhaps a symbol for the four cardinal points, the arches are to be understood as signs for the horizon and the rising and setting of the moon.

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