Rare Roman coin featuring Brutus up for auction in Geneva

"A numismatist's eyes light up when beholding a coin like this," because this is "a piece of history," said Frank Baldacci, head of the Numismatica Genevensis (NGSA) auction house behind the sale.

The aureus, the gold coin currency used in ancient Rome, was issued by Brutus and his friends a year or two after they assassinated Roman ruler Caesar in 44 BC, he said.

The coin, which weighs eight grammes (0.3 ounces), is "of immense historical importance both artistically and politically", NGSA said.

Bidding will start at 750,000 Swiss francs ($852,000), but Baldacci said it would likely go for more than a million francs.

"This could go quite high," he told AFP, pointing out that the piece was a bit like "the Da Vinci of Roman coins".

This particular aureus features Brutus's profile framed with laurel leaves on one side. The other celebrates his recent military victories with warlike symbols.

It is one of only 17 known examples.

The coin was "cast not in Rome but in a mint that travelled with Brutus and his armies as he tried to seize power after the assassination of Julius Caesar", Baldacci said, adding that it had a "propaganda value".

The laurel wreath is clearly a sign of "someone who wants to promote himself as emperor", he said, pointing out that "IMP" -- for emperor -- was engraved next to Brutus's face, even though he did not hold that title.
'Exceptionally rare'
The coin was struck shortly before the fateful Battle of Philippi in 42 BC, which ended with Brutus killing himself after losing to Octavian and Mark Antony as they sought to avenge Caesar's death.

The aureus has since travelled discreetly through the centuries, with little known about its owners along the way.

"But we know that during the Renaissance, there were a lot of princes and lords who had Roman coin collections," Baldacci said.

The coin resurfaced in the 1950s when it was catalogued as part of a private collection.

It reappeared again in 2006, when it was put up for auction in Zurich, and sold to a private collector for 360,000 Swiss francs.

"This is a coin returning to market after a generation," Baldacci said.

"These are exceptionally rare coins and the possibility to acquire them is also rare."

The coin is encased in an airtight box to prevent it from being altered, and to "guarantee its authenticity", Baldacci said.

Certification by specialists, he said, is done by among other things comparing a coin with other ancient coins and by examining the gold used.