The bronze ornament had been on display in the 'Pompeii and the Greeks' exhibition at the ancient site. It measured 7.3cm in diameter and had been loaned from Potenza's archaeological museum.
"The theft is a wound to Pompeii and to Italy's cultural heritage," said the site's manager, Massimo Osanna. "Although it isn't a piece of extreme monetary value, it also affects me from a personal point of view, as it came from an area I had been excavating."
Police have closed off the entire exhibition area to investigate the theft, which they believe took place at around 8pm on Wednesday.
The exhibition has seen around 600 finds from Pompeii and other cities such as Sorrento and Herculaneum put on display, all dating back to the Greek era, and was organized by the University of Campania together with the site's archaeological authorities.
Visitors taking ancient relics as 'holiday souvenirs' or even to sell at a profit is a recurring problem at Italy's cultural sites, and at Pompeii alone, guards have caught several tourists trying to smuggle out relics including mosaic tiles and parts of statues over the years. One tour guide previously told The Local the site is too easy to steal from due to too few custodians guarding the valuables.
Sometimes however, the thieves try to make amends. In 2014, a Canadian tourist returned an artefact she had stolen from Pompeii's amphitheatre during her honeymoon - 50 years earlier.
And several thieves have sent back the loot claiming the relics are 'cursed'. Pompeii's archaeological superintendent said in 2015 that he was considering setting up an exhibition of the returned artefacts and accompanying letters, to tell the story behind the stolen pieces.
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Photo: Tiziana Fabi/AFP