Lawyers for Israeli diamond magnate Beny Steinmetz urged a Swiss appeals court on Monday to throw out testimony from a former first lady of Guinea that contributed to his conviction for corruption.

The case involves an alleged plot, dating to the mid-2000s, in which Steinmetz’s BSGR Group squeezed out a rival for mining rights for vast iron ore deposits in Guinea’s southeastern Simandou region. The case has exposed the shady and complex world of deal-making and cutthroat competition in the lucrative mining business.

The prosecutor’s office has argued that from 2005 onward, Steinmetz crafted a pact of corruption with Guinean President Lansana Conte, who ruled the West African country from 1984 until his death in 2008, and with Mamadie Toure, his fourth wife, involving the payment of nearly $10 million.

Appearing before a Geneva appeals court on Monday, Steinmetz’s lawyer, Daniel Kinzer, said the terms and circumstances of a deal between Toure and the FBI in the United States were unclear, and defense lawyers never had a chance to question her — depriving Steinmetz of the chance for a fair trial and the right to cross-examine her, he said.

He said Swiss state prosecutors had “deliberately” excluded defense teams from any pretrial questioning of Toure in the United States, where she lives. She has reached an agreement with U.S. authorities in the case.

Toure did not appear for the original trial in January last year. At its conclusion, Steinmetz was sentenced to five years in prison and ordered to pay a 50-million-Swiss franc ($51.5 million) fine. Two other defendants received lesser penalties.

“It’s easier to falsely accuse a defendant when you don’t have to look at them,” Kinzer said. “The defense team was never able to cross-examine Madame Toure.”

He said a “face-to-face confrontation” was required under both Swiss law and from rulings by the European Court of Human Rights.

Geneva state prosecutor Yves Bertossa countered, however, that such accounts were admissible, but that they had to be used “with a certain prudence.” he said it was an “extraordinary” insinuation that U.S. or Swiss prosecutors might be “in cahoots” with Toure, and said that other material elements to the case — like written contracts, bank statements, and evidence from wiretaps — were sufficient to uphold conviction.

The appeal is expected to run through Sept. 7. A verdict is expected at a later date.

Backers of the Israeli tycoon insist that the lower court didn’t get a full understanding of the facts of the case, and believe that the court wanted to set an example that Switzerland — which has had a reputation over the years for secretive financial dealings — can hold financial kingpins to account when necessary.

After the verdict, Swiss transparency group Public Eye hailed a “landmark ruling” that showed the court could see through a “slick” legal defense.

Steinmetz, 66, has denied the charges and remains free pending the appeal. If the conviction is upheld, his lawyers can appeal to the Swiss federal court. He was in court Monday.

In its court filing, the prosecutor’s office said BSGR won exploration and exploitation licenses in Guinea between 2006 and 2010 in Guinea’s southeastern Simandou region, while its competitor — Anglo-Australian mining group Rio Tinto — was stripped of its mining rights on two sites in the region.

Steinmetz’s defense team says a mountain range in the area holds some of the world’s largest untapped deposits of iron ore, and the standoff has stifled any hopes to reap them — and offer a potential windfall for an impoverished country. They say BSGR was the first company to study the feasibility of mining iron ore in the area.

(AP)