He dug down 4ins to find what he initially thought was a small gold button, only to turn it over to reveal the depiction of a helmeted male bust.
He started shaking through excitement and showed his friend the coin before putting it safely in his pocket.
The coin, which measures half an inch in diameter and weighs 1.3grams, is one of just eight known examples of the 'Crispus' thrymsa.
