The thieves broke in overnight Friday and managed to overcome the “sophisticated security system” at the Museum of Fourviere in the city of Lyon, the museum said in a statement.
They got away with the Crown of the Virgin, the centrepiece of the collection, which was created in 1899 with 1,791 precious stones and pearls gifted by well-to-do Lyonese families of the day.
The value of the piece was put at “a little over a million euros”. Each gemstone was painstakingly logged last year by a team of researchers, the museum said, a fact that will help trace each part of it.
The robbers got away with two other pieces from the museum's permanent collection, a ring and a chalice.
The museum, situated on a hill next to a basilica, was closed as investigations continued.
The museum displays treasures from the basilica which are representative of the work of goldsmiths in the nineteenth and twentieth centuries.