Cry for the Baron

Each previous owner of 'The Diamond of Tears' had been tortured and met a murderous end. Jacob Bernstein was the fifth in line and was to become a victim, which resulted in John Mannering (aka 'The Baron') beginning an investigation which proved to be intriguing and dangerous. What is it about this priceless jewel that attracts death for its owners, and why is 'The Baron' himself now in danger?