The man, who doesn't want to be named, made the discovery in Victoria's goldfields - which were the heart of Australia's gold rush in the 1800s.
Darren Kamp, who valued and bought the specimen, said it was the biggest he'd seen in his 43-year career.
"I was just gobsmacked... It's a once in a lifetime find," he told the BBC.
Kamp hadn't thought too much of it when a man wearing a large backpack walked into his prospecting store in Geelong, about an hour south-west of Melbourne.
