The prehistoric monument of Stonehenge is shrouded in unanswered questions, including where the site's stones came from. There are two types of stones that make up the iconic monument, smaller bluestones and larger sarsen sandstones. The main architecture is comprised of these sarsens, each weighing around 20 metric tons (the same weight as about 10 cars).

Recent research published in Science Advances traces the origins of the sarsens by using portable X-ray fluorescence spectrometry to chemically "fingerprint" each of these large boulders. Bombarding a spot on the sarsen with X-rays makes an invisible glow radiate back. By measuring this glow, scientists can figure out exactly what elements are in the stone. 

Sarsens are mostly silica (the stuff in sand and computer chips), but trace amounts of elements such as aluminum, phosphorus, and calcium are clues to their origin. Scientists were able to match the chemical fingerprints of 50 out of 52 sarsens to stones from West Woods, Wiltshire, about 25 km north of Stonehenge. Now two questions remain: where did those other two stones come from, and why were they sourced from a different place?