New Delhi, June 27 -- In May, Roberto Diaz performed at the US Library of Congress using a rare viola made by Antonio Stradivari in 1690. Called the Tuscan-Medici viola, it is now valued at $30 million, possibly the world's most expensive instrument.

Diaz, who leads the Curtis Institute of Music in Philadelphia, has played the viola for nearly 15 years. He even recorded a Grammy-winning album with it. Many admire its fine grain and craftsmanship, calling it both simple and deeply beautiful.

According to Diaz, the instrument produces an "incredibly resonant but kind of compact sound".

"It has a tremendous brilliance to it, and you can hear this. People call it the 'Cremonese spin' inside the sound, which is what makes the sound just kin...