In archaeological notation it is simply Stone 80. It lies near the geometric centre of Stonehenge, oriented along the monument's main axis, and weighs about six tons. If it ever stood upright, it would have reached nearly two metres tall. Some specialists believe it was always recumbent, deliberately placed flat as a kind of low altar or hearth. Its name comes from a comment by the architect Inigo Jones, who surveyed the monument in 1620 and wrote, with characteristic seventeenth-century caution, whether it might be an Altar or no I leave to the judgment of others. The phrase stuck. For four centuries everyone has called it the Altar Stone.

Stone 55, one of the great sarsen megaliths from the inner trilithon horseshoe, fell across the Altar Stone at some point in antiquity. It lies perpendicular across Stone 80 now, pinning it down. The collapse may explain why no one has properly excavated the Altar Stone since the 1950s. Even that dig left no written records that survive, and there are no samples in any museum that can be confirmed as coming from the slab itself. Every analysis of its composition has had to work from fragments believed to belong, or from in-situ measurements made when no one was allowed to chip away at the source.

For decades the standard story said the Altar Stone was Old Red Sandstone, probably from the Senni Beds of south Wales, transported the same general route as the bluestones from the Preseli Hills. Its purplish-green micaceous sandstone fit broadly with the Welsh geology. Then in 2024 a team led by researchers from Curtin University in Australia and Aberystwyth University in Wales published a study in Nature comparing the Altar Stone's age fingerprint, the radiometric signatures locked in its zircon and apatite grains, with sandstones across Britain. The match was not in Wales. It was in the Orcadian Basin, the great sedimentary trough that runs through Caithness, Sutherland and Orkney in the far north-east of Scotland.

How a six-ton slab travelled from north-east Scotland to Wiltshire in the Neolithic remains an open question. Overland would have crossed mountain ranges and dense forest. By sea, around the entire British coast, would have demanded a vessel and a crew capable of working open Atlantic swell with a stone too heavy to portage easily. Whichever route, it implies a network of relationships across the whole island five thousand years ago, more ambitious than any modern reconstruction of Neolithic society has yet allowed. The Altar Stone has been lying quietly at the centre of Stonehenge since around 2600 BCE. In 2024 it abruptly became the most important single object in British prehistory, simply because it forced everyone to admit they had underestimated the people who put it there.