THEY are among the most mysterious artefacts ever discovered in Scotland, ornate relics whose purpose has long been lost to the mists of time.

For decades archaeologists have been baffled by the discovery of carved stone balls from sites once used by the ancient people of Scotland, dating back as long as 5,000 years ago.

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Mostly carved with distinct ‘knobs’ - usually numbering three - some are highly decorated with swirls and spirals while others are rough and barely finished.

Many different theories have been proposed to explain why they were carved, ranging from the mundane to the bizarre.

It has been suggested they could be weights for fishing nets, or the heads of a mace-like weapon. Some scholars have said the stones, which mostly fit in the palm of an adult's hand, could be part of some form of ball game, or they could even be the 'ballbearings' used to erect the rings of standing stones which also appeared during this time.

HeraldScotland: Picture of The Ring of Brodgar, Mainland Orkney...I hope you will consider this picture for The Big Picture Feature in your newspaper..The picture was taken in December 2013 , mid afternoon..The Orkney Islands are a great location for Photography, with ma

Carved stones have been found at the Ness of Brodgar

However, none of these ideas explains why the balls have been carved so intricately or why so few show signs of wear and tear.

Now one researcher has come up with a new theory, inspired by similar objects from the modern age.

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Author and amateur historian Jeff Nisbet said that the time and care lavished over the stones may mean they are a sort of 'apprentice piece' created by neolithic stone masons to show off their skill.

The Scottish-American writer, who was born in Edinburgh before emigrating to the US aged 11, says that the stones could have been a way for ancient builders to demonstrate how they had mastered their trade, much in the way portfolios work today.

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Mr Nisbet said: "A Neolithic stonemason, would have needed some other type of certification, and an easily portable carved stone ball could have eminently suited that purpose.

"Besides the symbol stones and stone circles, there would have been houses and walls to build, cist burial slabs to cut, and tools and weapons to make -- all practical and marketable uses for the skilled stonemason’s craft.

"Unlike the fishermen who sold their catch at the harbour, or the farmers who brought their livestock to the local market, stonemasons would have often been required to travel from job site to job site, and would have to prove they had the skills to handle the tasks at hand."

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Mr Nisbet added: "Like the résumés and portfolios of today’s workforce, the carved stone balls of the ancient stonemasons would be visible and tangible testimony of the work they were qualified to do."

Around 400 of the stones have been found in Scotland, including examples from Skara Brae and the Ring of Brodgar on Orkney, and examples can be found in both Kelvingrove Museum in Glasgow and the National Museum of Scotland in Edinburgh.

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The Towie Ball

Among the finest found is the 'Towie ball', uncovered on Glaschul Hill, Aberdeenshire, in 1860, which is carved with a design which is echoed in the Neolithic tomb ar Newgrange in Ireland.

The carved balls were created for thousands of years and may have held ritual significance for the Neolithic people of Scotland. One popular theory is that they were used at tribal gatherings as 'speaker stones' - held by those with the right to address the group.

Mr Nisbet added: "Once cupped in ancient hands, these humble yet very personal objects can still give us a glimpse of who their makers were, bring us the curiously comforting knowledge that these craftsmen were not so very different from ourselves, and show us that even in our widely separated worlds our lives, indeed, may continue to play out on common ground."