The connecting means (14) for the stones have a rail element (14) enclosed by prismatic guide faces (18,19). The guide faces are associated with the ring rail (11) and the rail element (14) is associated with the stone (16). The prismatic guide faces form the opposing side faces of a recess and are arranged transversely to the ring rail (11) and interrupt same only in part. Locking means such as ball and recess are provided between prismatic guide and rail elements.
