John Wyatt paid £50 for more than a ton of rocks. When he was cleaning them, he found one had a celtic cross carved into it on one side, and a bird-like beast on the other.
It's estimated to make £10,000 at auction.
John told the Daily Telegraph: "I was doing a bit of work in my own garden and saw an advert for some natural stone. I phoned the people up and went to collect it in my pick-up.
"When I got home I saw what I thought was the tail of the dragon on one of them. It was lucky I was looking.
"It looked like some of the things you see round her in museums so I contacted a museum and archaeologists got very excited.
"No one could really say exactly what it was but they knew it was important."
Mr Wyatt, who's 32, hopes to pay off some of his mortgage with the money raised at auction from the stone.
It'll be auctioned at Duke's, of Dorchester in Dorset.