Experts used medical MRI scanning technology to analyse the fossil to discover if there was a rare gem inside. And their results showed up a mysterious smooth object about the size of a golf ball. If it were removed and identified as a pearl, it could be worth many thousands of pounds.
But experts will not explore the contents any further because it would mean the fossil would have to be destroyed.
The oyster – which measures about 7in across – remains at the Blue Reef Aquarium in Portsmouth, where it is kept securely and brought out only for lectures.
Lindsay Holloway, from the aquarium, said: ‘It was discovered in the nets of a fishing boat which was dredging here in the Solent.
When the fishermen came back to port they thought it was real, but when they picked it up, cleaned it, and had a closer look they could tell it was a fossil. It had completely turned to stone.’
Jewellery expert Geoffrey Munn said: ‘To have a pearl the size of a golf ball would be exceptional. The biggest that have been recorded are about half that size.’
Following news of the discovery a company in Cheltenham called Cobalt, which provides state of the art scanning services to the NHS and private patients, volunteered to scan the oyster to see what was inside.
Lindsay Holloway added: 'A member of the public called and informed us it was on display at a local fishmongers so we called them and they gave it to us to have in the aquarium.
'Oysters can be aged by annual growth rings on their shells and we have counted more than 200 rings on this oyster making it an extremely long-lived individual.
'It's obviously a million-to-one chance that it would contain anything but, if you were to go purely on the dimensions of the shell then you'd be looking at a golf ball-sized pearl.'