A second-hand dealer in France has appeared in court accused of deceiving a couple by paying €150 (£130) for an African mask which he resold for €4.2m.
The pensioner couple found the mask from Gabon at the home of an ancestor who had been a colonial governor.
They sold it to the dealer in 2021, only to find out its true value following an auction six months later.
As the case opened on Tuesday, Gabon's government asked for proceedings to be halted and the mask returned.
The saga began when the couple - who are in their 80s and live in central France - asked the dealer to clear their holiday home near the southern town of Alès. The house had belonged to René-Victor Fournier, a colonial administrator in the early 20th Century.
The wooden mask was found in a cupboard. The dealer argues that he had no idea how valuable it was when he bought it.
In March 2022, reading about the auction in the city of Montpelier, the couple discovered that it was a rare 19th-Century "Ngi" mask made by the Fang people of Gabon.
The auction catalogue said it had been collected around 1917 by Fournier "in unknown circumstances".
One expert said at the time that only about 10 such items had ever been made by Fang masters. "This mask is rarer than a Leonardo da Vinci painting," he told French media.
The mask - which the auctioneers had initially valued at €300,000 - was bought for €4.2m by an unnamed bidder.
The couple then launched a civil case to annul the sale.
The Gabonese government has argued that the mask was stolen in the first place and should be returned home. It has asked for the court to delay its ruling pending a decision on its own complaint.
In 2020, the French parliament voted to return to Senegal and Benin prized artefacts that were looted during colonial times.
There are some 90,000 African artefacts in France, most from sub-Saharan Africa.