A new study exploring the origins of the six tonne Altar Stone at Stonehenge has ruled out a theory that it came from an area of Scotland.
The 4.9 metre sandstone slab within the circle was initially believed to have originated in Wales until a more recent study pointed towards it coming from north east Scotland.
Scientific tests were carried out in the Orcadian Basin area of Scotland, but researchers failed to find a location with the same characteristics of the stone in that area.
However study lead, Professor Duncan Pirrie from the University of South Wales, said the latest investigation has left them "far better equipped to focus on the areas most likely to yield an answer."
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Pirrie said his latest publication on the Altar Stone has "built the clearest picture yet" and was "the most comprehensive geological investigation of the Altar Stone ever undertaken".
Professor Nick Pearce from Aberystwyth University, who worked on the previous research, said "ruling out potential source areas is just as important as identifying new ones".
The geochemistry expert said: "Every location eliminated narrows the search and brings us closer to discovering where the Altar Stone originated.
"The stone is unique among those at Stonehenge, and tracing its source would provide valuable new insights into its remarkable journey to Salisbury Plain thousands of years ago."
The world-famous Wiltshire monument was constructed in multiple phases over thousands of years, between 3000 and 1600 BC. However, not all stones come from the area.
The smaller bluestones travelled around 225km from west Wales with research suggesting a few were taken directly from another circle.
The latest study combined new observations and looked at fragments of the stone in museum collections, examining structure, minerals and composition.
It has been published in the Journal of the Geological Society.
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