The origin of Stonehenge’s iconic standing stones has baffled archaeologists for hundreds of years.

Last month, the plot thickened as scientists made a bombshell discovery that the stone circle’s altar stone could be traced back to Scotland, rather than Wales.

Now, the mystery has deepened once again as a new study finds that this bluestone slab did not originate from Orkney as most experts had ᴀssumed.

‘The mystery of where the stone came from is becoming clearer and clearer as we begin to rule out specific areas in north-east Scotland,’ said Professor Richard Bevins from Aberystwyth University, the lead author of the new study.

‘This research is radically changing our thinking about the origins of the Altar Stone. It’s thrilling to know that our chemical analysis and dating work is slowly unlocking this great mystery.’

Scientists have long known that Stonehenge’s smaller ‘bluestones’ were transported to the Salisbury Plane from the Preseli Hills in Wales, around 240km (150 miles) away.

However, one of the circle’s bluestones – the Altar Stone – has always stood out as unique.

Professor Bevins explained: ‘The Altar Stone is anomalous in many ways to both the bluestones and the sarsens at Stonehenge.

‘Whilst, at six tonnes, it is nowhere near the size of the sarsens, it is substantially larger than the bluestones, with which it has previously been classed.’

Previous researchers had ᴀssumed this strange stone had come from the same area as the other bluestones, but recent analysis of the age and chemistry of the altar stone revealed that it could not have come from Wales.

Rather, experts found a remarkable similarity with the Old Red Sandstone of the Orcadian Basin in northeast Scotland.

The team said they could conclude with 95 per cent accuracy that the stone came from this area – which encompᴀsses parts of Inverness, Thurso, Orkney and parts of Shetland.

After the discovery of the Scottish connection, most researchers ᴀssumed that the stone would likely originate from this archipelago.

This is because of well-documented communication between the cultures of the Salisbury Plane and the Orkney Islands in 3,000 BC – the date of Stonehenge’s earliest construction.

Professor Bevins told MailOnline: ‘Orkney was a major cultural centre in Neolithic times, as indicated by its profusion of monuments and buildings dating from that time.

‘There was clearly long-distance communication between these Neolithic communities. In addition, the geology of Orkney was right for a possible match to the Altar Stone. So Orkney seemed a logical place to investigate.’

The researchers analysed the chemical and mineralogical composition of the stones in Orkney’s two biggest stone circles: the Stones of Stenness and the Ring of Brodgar.

They also used portable X-ray devices to analyse field samples of rock deposits across a number of Orkney’s islands.

Contrary to what they had ᴀssumed, their analysis revealed that the Stonehenge altar stone and those on Orkney had strikingly different makeups.

Researchers are now left searching for the origins of the altar stone once left.

But by ruling out Orkney so quickly, they can now focus their efforts elsewhere.

Professor Bevins said: ‘As an academic, I have been fascinated by Stonehenge for decades. I and my other colleagues in the team will continue to work to pin down where exactly in the north-east of Scotland the Altar Stone came from.’

And, although disproving the Orkney theory may be a setback, Professor Bevins told MailOnline he remains positive the stone will be found.

‘I am optimistic that the source of the Altar Stone will be found. However, the Orcadian Basin covers a large area in north-east Scotland so it might take some time to locate the source,’ he added.

‘But we have some clues in terms of the mineralogy and geochemistry of the Altar Stone so we are not going in blind.’

The Stonehenge monument standing today was the final stage of a four part building project that ended 3,500 years ago

Stonehenge is one of the most prominent prehistoric monuments in Britain. The Stonehenge that can be seen today is the final stage that was completed about 3,500 years ago.

According to the monument’s website, Stonehenge was built in four stages:

First stage: The first version of Stonehenge was a large earthwork or Henge, comprising a ditch, bank and the Aubrey holes, all probably built around 3100 BC.

The Aubrey  holes are round pits in the chalk, about one metre (3.3 feet) wide and deep, with steep sides and flat bottoms.

Stonehenge (pictured) is one of the most prominent prehistoric monuments in Britain

They form a circle about 86.6 metres (284 feet) in diameter.

Excavations revealed cremated human bones in some of the chalk filling, but the holes themselves were likely not made to be used as graves, but as part of a religious ceremony.

After this first stage, Stonehenge was abandoned and left untouched for more than 1,000 years.

Second stage: The second and most dramatic stage of Stonehenge started around 2150 years BC, when about 82 bluestones from the Preseli mountains in south-west Wales were transported to the site. It’s thought that the stones, some of which weigh four tonnes each, were dragged on rollers and sledges to the waters at Milford Haven, where they were loaded onto rafts.

They were carried on water along the south coast of Wales and up the rivers Avon and Frome, before being dragged overland again near Warminster and Wiltshire.

The final stage of the journey was mainly by water, down the river Wylye to Salisbury, then the Salisbury Avon to west Amesbury.

The journey spanned nearly 240 miles, and once at the site, the stones were set up in the centre to form an incomplete double circle.

During the same period, the original entrance was widened and a pair of Heel Stones were erected. The nearer part of the Avenue, connecting Stonehenge with the River Avon, was built aligned with the midsummer sunrise.

Third stage: The third stage of Stonehenge, which took place about 2000 years BC, saw the arrival of the sarsen stones (a type of sandstone), which were larger than the bluestones.

They were likely brought from the Marlborough Downs (40 kilometres, or 25 miles, north of Stonehenge).

The largest of the sarsen stones transported to Stonehenge weighs 50 tonnes, and transportation by water would not have been possible, so it’s suspected that they were transported using sledges and ropes.

Calculations have shown that it would have taken 500 men using leather ropes to pull one stone, with an extra 100 men needed to lay the rollers in front of the sledge.

These stones were arranged in an outer circle with a continuous run of lintels – horizontal supports.

Inside the circle, five trilithons – structures consisting of two upright stones and a third across the top as a lintel – were placed in a horseshoe arrangement, which can still be seen today.

Final stage: The fourth and final stage took place just after 1500 years BC, when the smaller bluestones were rearranged in the horseshoe and circle that can be seen today.

The original number of stones in the bluestone circle was probably around 60, but these have since been removed or broken up. Some remain as stumps below ground level.