The next development was in 1000 BC, when China started making bronze and copper cowrie shells. It wasn't long before the very firsr coins often had holes in them so thar people could pass a piece of string through them to keep them together. By 500 BC metal coins had begun to appear in countries like Persia and Greece, and later in the Roman Empire. These were usually lumps of silver with the heads of various gods and emperors stamped on them to show they were real.