It has long been supposed that rhyolites from the northern Preseli Hills helped build the monument. But research by National Museum Wales and Leicester University has identified their source to within 70m (230ft) of Craig Rhos-y-felin, near Pont Saeson. The museum's Dr Richard Bevins said the find would help experts work out how the stones were moved to Wiltshire.
For nine months Dr Bevins, keeper of geology at National Museum Wales, and Dr Rob Ixer of Leicester University collected and identified samples from rock outcrops in Pembrokeshire to try to find the origins of rhyolite debitage rocks that can be found at Stonehenge.
By detailing the mineral content and the textural relationships within the rock, a process known as petrography, they found that 99% of the samples could be matched to rocks found in this particular set of outcrops.
Rhyolitic rocks at Rhos-y-felin, between Ffynnon-groes (Crosswell) and Brynberian, differ from all others in south Wales, they said, which helps locate almost all of Stonehenge's rhyolites to within hundreds of square meters.
Within that area, the rocks differ on a scale of metres or tens of metres, allowing Dr Bevins and Dr Ixer to match some Stonehenge rock samples even more precisely to a point at the extreme north-eastern end of Rhos-y-felin. Dr Rob Ixer of Leicester University called the discovery of the source of the rocks "quite unexpected and exciting".