A 600-year-old Chinese coin was found on an island just off the coast of Kenya, researchers at Field Museum in Chicago announced Wednesday.

The finding resulted of a joint expedition led by Chapurukha Kusimba of the museum and Sloan Williams of the University of Illinois-Chicago.

“The coin proves trade existed between China and eastern Africa decades before European explorers set sail,” said Kusimba.

Made of copper and silver, the coin is a true rarity. It boasts a square hole in the center so it could be worn on a belt. Researchers believe it was issued by Emperor Yongle of China -- the third emperor of the Ming Dynasty in China, reigning from 1402 to 1424 – and his name is written on the coin.

Researchers from Kenya, Pennsylvania and Ohio participated in the expedition. Together with the coin they also found human remains and other artifacts predating the coin.