The works included a Courbet painting, Chagall and Matisse lithographs, and a 2.08-carat diamond.
Workers tasked with transporting the objects allegedly stole them and then later sold some of them with the complicity of auctioneers.
Nearly all of the 50 defendants involved in the case at the Hotel Drouot, the biggest, oldest and most famous auction house in France, appeared in a Paris court today charged with gang-related theft, conspiracy to commit a crime or handling stolen goods.