A metal cup acquired by a rag and bone man from Somerset has been revealed to be a pure gold goblet from the 3rd or 4th Century BC, worth up to £500,000.
The vessel has two female faces looking in opposite directions with their foreheads decorated with a snake motif.
William Sparks, who ran a scrap metal business in Taunton during the 1930s and 40s, left the ancient treasure to his young grandson John Webber.
The goblet is to be sold by Duke's auction house in Dorchester, Dorset.
Mr Webber, 70, said: "My grandfather was originally a proper rag and bone ban from Romany stock and lived in a caravan.
"I sent it to the British Museum and the experts there hadn't seen anything like it before and recommended I had it tested at a laboratory," said Mr Webber.
The analysis confirmed its age and that it had been painstakingly crafted from just one piece of gold.
Peter Northover, the scientist who reported on the gold analysis, said in the report: "The method of manufacture and the composition of the gold are consistent with Achaemenid gold and gold smithing."
The Achaemenid empire was based around Persia, but at its height stretched from Iran to Libya. It was wiped out by Alexander the Great in 330 BC.