The Diary and the Ring: Destroyed Mystery - Lord Voldemort's Horcruxes
Pages 500-503
Dumbledore tells Harry that two Mystery - Lord Voldemort's Horcruxes have already been destroyed: Tom Riddle's diary, which Harry destroyed in his second year, and Marvolo Gaunt's ring, which Dumbledore himself destroyed, though at great personal cost. He shows Harry his blackened hand, which was cursed in the process of destroying the ring.
