Edvin Marton performes on Antonio Stradivarius violin from 1697. What is the secret of a Stradivarius violin, the world's most exquisite and valuable musical instrument? For centuries, musicians and music lovers have often attributed a Strad's magnificent, other-worldly tone--it has been likened to the voice of the human soul--to a secret-formula varnish used by Antonio Stradivarius three centuries ago in his stringed-instrument workshop in Cremona, Italy.