Archaeologists at Oram's Arbor have unearthed their most important find yet-- a medieval stone engraved with an image likened to that of St George slaying a dragon.

The bright green stone, which bears the image of a figure on horseback lancing a serpent, was discovered at the Winchester dig by volunteer, Robin Tuff.

Originally thought to be Roman because of the classical style of the engraving, experts now believe it is much more recent, dating to the 11th or 12th centuries.

It would have been set in a signet ring and the image used as a stamp or seal. The stone is thought to be Byzantine and could be a relic of the Crusades.

Spending four days working at the site alongside a team of professional archaeologists, Mr Tuff (62), of Stockbridge Road, uncovered the stone in a main trench. It was about a foot beneath the surface.

Describing the find as "unexpected," he said: "We were just cleaning up the chalk when I suddenly saw what I though was a bright green button, so I hauled it out.

"At that stage, we weren't really looking for anything. It was very bright, a kind of jade green. But now it has been out of the ground a few days, it has got darker. Being so far down, it was protected."

The site was once part of an Iron Age enclosure, dating to c. 100BC. Excavation started in June after foot and mouth restrictions forced students from King Alfred's College to cancel a dig in Somerset. Learning of the problem, Winchester City Council offered Oram's Arbor as an alternative site.

The city's museums service took control of the dig in mid-July, as part of the annual community archaeology programme, involving about 15 Winchester people every day, mainly from the city's Fulflood area.

Along with his wife, Gerry, Mr Tuff spent four days as a volunteer. "There is a good community down there and I would definitely do it again. But you never know quite what you are going to find."

Site director, Steven Teague, said: "The dig is going well and we have recovered a wealth of objects, although the vast majority of the volunteers have never been on an archaeological site before."

Around 200 people visited on Sunday when Winchester Museums Service held an open day. Updates can be found on the internet, at www.wkac.ac.uk/departments/archaeology or www.winchester.gov.uk/heritage/communityhtm