The plaintiffs accuse the local farmer of mining the precious stones on their land.

Subsoil in the volcanic region of central France is saturated with gems.

The farmer, who owns property on the opposite bank to the plaintiffs, has been mining for stones near the town of Issoire with a partner.

But his neighbours accuse him of also digging for the precious stones on their side of the river.

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Their lawyer Patrick Roesch said they want at least €1 million in compensation.

“The case began about a year ago when my clients began to wonder about the frequent use of a path running alongside their property,” Roesch said ahead of the hearing on Tuesday.

After watching a television report on sapphire mining in Auvergne, the couple recognised the stream at the bottom of their property and realised that the prospectors had regularly worked the deposits.

“Some of them even boasted of having found the most beautiful and largest precious stones — sapphires in this case — on the part of the riverbed belonging to my clients,” Roesch said.

The river does not belong to the state, so if two banks belong to different owners, then under French law each of them owns half of the riverbed.

A lawyer for the prospectors declined to comment when contacted by AFP.