The new discovery - which will take pride of place in Shetland Museum and Archives' Pictish display. © Shetland Museum and Archives

An intriguing discovery of a carved Pictish stone has been made in Shetland, at a site known for yielding Pictish treasures.

The sculptured stone engraved with mysterious symbols – identifiably Pictish – was turned up in Cunningsburgh, where another carved stone was found in 1992. Shetland, though somewhat isolated then as now, was part of the Pictish kingdom which stretched across northern Scotland in the country's pre-Christian era from 400-800AD.

"It was found by the gravedigger," explained Dr Ian Tait, Curator of Collections at Shetland Museum and Archives. "The place where it was found has been a graveyard since at least the Middle Ages, and before that it was almost certainly a centre of power."

"The gravedigger was fortuitously the same man who found the other stone in 1992, so although he's not an archaeologist he knew something was afoot when he saw this one and he brought it to me in the museum."

"I was absolutely flabbergasted," he continued. "It's a once in a lifetime find and he's found two!"

The slab measures 45 by 25cm (18 by 11 inches), and is a portion that has broken off from a larger stone. It probably dates to about 700AD.

One side of it bears known Pictish symbols such as a Z-shaped figure with ornate terminals. There are also two disc shapes on either side of this symbol, making it consistent with other Pictish carvings that have been found. However, the two discs on this stone have crosses inside them – a novel finding.

The stone found in 1992. © Shetland Museum and Archives

No-one knows the significance of Scotland's 40-plus Pictish symbols, but the same motifs are found on various stones and have been given names. The 'double disc and Z-rod' is one of the commonest, with the discs often containing circles or spirals – but the latest discovery is the only one with this type of cross and is a first in Shetland.

While the crosses could simply be geometric designs, Ian Tait suggests that the crosses indicate the mingling of indigenous Pictish religious beliefs with the incipient Christian faith.

"The crosses strongly indicate a Christian symbol, from a period of transition from the Pictish religion to Christianity," he said.

The most similar crosses on other Pictish findings in Shetland are on the Papil Stone, but the latest find is really special.

The site where the stone was found has been a centre of important activities down the ages. Though it is not known whether there are burials there dating from the Iron Age, three parts of gravestones have been found with inscriptions in Norse runes – demonstrating that the Vikings were active in the area – and four stones have been found with inscriptions in the ancient language of Ogham.

The impressive St Ninian's Isle Treasure includes jewellery and decorated bowls. © National Museums Scotland

It is likely the site was a centre of cultural or political power to the Picts, too. The last Pictish find, from 1992, depicts a figure in a dog-head mask.

Another interesting thing about the new find is that it bears some 18th century graffiti – an inscribed date of 1769 – meaning it must have been above ground then. No-one knows when the stone was buried, or who by. It's even possible it was deliberately broken by the pagan Vikings, who would have paid no respect to Pictish artefacts or beliefs.

"There are almost certainly more stones down there," said Ian, "but it's a working graveyard so there's no chance that archaeologists will excavate it. We will be relying on more chance finds – which adds to the excitement!"

The new stone will be on show at Shetland Museum and Archives throughout June 2008, in the run-up to the display of the St Ninian's Isle Treasure, and will then form a key part of the re-displayed Pictish exhibits.

The St Ninian's Isle Treasure is on special loan to Shetland from the Scottish National Museum, to mark the 50th anniversary of its discovery on the Shetland island. The Pictish silverware will be on show for three months from July 4 2008.