Suspected souvenir hunters broke into Stonehenge and vandalised the ancient monument, English Heritage has said.
A hammer and screwdriver were used to take a small chip the size of a 10p piece from the side of the Heel Stone.
English Heritage said further damage was prevented by security guards who spotted the two men at the 5,000-year-old site in Wiltshire.
Police believe the vandals could be the same two people caught on CCTV acting suspiciously a few days earlier.
English Heritage said souvenir hunting was once a legitimate practice and chisels used to be handed out to people visiting the stones.
The recent act of vandalism which happened on the evening of 15 May at about 2200 BST is thought to be the first attack on the site for many years.
A spokesman from Wiltshire Police said, "Damaging an ancient monument is a serious offence and we take this type of damage extremely seriously. We are determined to identify the offenders and bring them to justice.
"As this is a tourist area we would also be interested in anyone who may have taken photographs of this incident."