Back in the 1950s, a chunk of rock went missing from the magical tumble of megaliths that now compose Stonehenge. The chunk, a three-and-a-half foot cylindrical core, had been drilled out of one of the site’s massive sarsen stones during repairs and taken home by an employee of the diamond-cutting firm that carried out the work.

That core was recently returned to archaeologists after 60 years, and it is proving to be a pivotal piece of evidence for locating the source of these sarsen stones that make up Stonehenge.

Geologists determined nearly a century ago that the bluestones were dragged, carried or rolled to Stonehenge from somewhere in the Preseli Hills in western Wales, some 180 miles away. Last year a team of archaeologists led by Michael Parker Pearson of University College London revealed evidence of the exact location of two of the quarries. As for the sarsens, conventional wisdom holds that they derived from deposits on the highest points of the Marlborough Downs, 18 miles north of Stonehenge.

But that has recently been disproved by David Nash, a geomorphologist at the University of Brighton, and his team of researchers. Dr. Nash has traced the source of almost all the sarsens to West Woods, on the southern edge of the Downs, and several miles closer to Stonehenge. His team analyzed the geochemical fingerprint of the 52 sarsens that remain at the ancient site.

The breakthrough came last summer when the long-lost core that we mentioned earlier was returned to English Heritage, the charity that manages Stonehenge. The sarsen cylinder offered Dr. Nash the unique opportunity to analyze a sample unaffected by surface weathering, which can slightly alter the chemical composition. Drilling through the ancient stones is now discouraged, so this was a rare opportunity.

To determine its chemical makeup, researchers used a variety of noninvasive spectrometry techniques. Once the geochemical signature was established, they sampled sarsens from 20 locations across southern England, including six on the Downs. A data set comparison resulted in a single match, West Woods.

For archaeologists, the discovery solves a big piece of the mysterious Stonehenge puzzle.