Looking for some natural stone for a rockery in his garden, John Wyatt thought he had found a bargain when he saw a job lot advertised for £50.
He was more right than he knew. For when he took the ton and a half of rock home he discovered that it contained an ancient stone carving worth thousands of pounds.
Mr Wyatt, 32, was cleaning mud and moss off the pieces when he spotted one with a Celtic cross carved on one side and a mythical birdlike beast on the other.
He had the 21 by 15in piece examined by an expert, who told him it dated from Anglo-Saxon times.
It is believed to have once formed part of a cross-slab from an early Christian monument.
It is possible that it was smashed by Viking invaders in the 9th century, in a deliberate act of desecration against Britain’s Christian population.
The rock is now being sold at auction with a pre-sale estimate of £10,000.
Mr Wyatt, of Chester, said: “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. There must have been a ton and a half and I paid about £50 for the lot.
“The stones were covered in mud and moss and 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.
“I cleaned it off and realised it was carved. It looked like some of the things you see round here in museums so I contacted a museum and the archaeologists got very excited.
“No one could really say exactly what it was but they knew it was important.”
He intends to pay off part of his mortgage if and when it is sold.
Guy Schwinge, an auctioneer, said: “The Anglo-Saxon stone is an important find and the stylistic vocabulary on the cross is indicative of an Anglo-Saxon origin and it probably dates from the 9th or 10th century.”
Also going under the hammer at the same sale is a Roman sarcophagus that for years acted as a plant pot in an Oxfordshire garden. The estimate for that is £25,000.
Mr Schwinge said that the sarcophagus dates from the 2nd or 3rd century AD and, although damaged, remains a rare and important find.
Made from white marble, it depicts two river gods holding horns of plenty while reclining on the back of dolphins and flanked by palm trees.
In the centre is Cupid embracing a mourning figure, who in turn is holding a quiver of arrows.
Mr Schwinge said: “We can only speculate on how this important Roman artefact ended up in an Oxfordshire garden, but in all probability it was brought back in the 18th century by a gentleman on the Grand Tour.
“It had been used for bedding plants to bring a bit of colour to the garden.
“Both these lots [1139 and 1140] show just what value can be found in gardens across the country.”
Both pieces are being sold in Dorchester, Dorset, on Friday.