In 'Harry Potter and the Chamber of Secrets,' Harry destroyed the Horcrux (Tom Riddle's diary) by stabbing it with a Basilisk fang. The Basilisk was a giant serpent whose venom and fangs were capable of destroying Horcruxes - one of the few substances powerful enough to do so. When Harry stabbed the diary with the Basilisk tooth, it began to bleed ink and the soul fragment within was destroyed, effectively killing the memory of Tom Riddle that inhabited the diary.