Archaeologist Tomos Jones says pieces of Preseli bluestone have been put up for sale online
Plans to tackle heritage crime have been extended to protect historic sites across Wales.
Archaeologists, national park officials and Dyfed-Powys Police officers have held a meeting to discuss growing concerns about vandalism.
At the conference in Llandrindod Wells, Powys, areas at risk of damage were outlined and training was given on how to best protect remote monuments.
Police, Cadw wardens and park rangers will regularly patrol sites.
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