Marvolo Gaunt's ring

Gaunt's Ring was an heirloom of the Gaunt family, owned originally by Marvolo Gaunt, descendent of Salazar Slytherin. The signet ring passed from generation to generation, until it was stolen from Morfin Gaunt by Tom Riddle.

While at Hogwarts, Tom openly wore the ring. He later used the ring, as an heirloom of the Slytherin family, as one of his horcruxes. Tom did not enchant the ring right away, as he was seen wearing it while asking Horace Slughorn about horcruxes, as seen through Slughorn's memories through a pensieve.

Powers
Besides being a horcrux, the ring was apparently cursed. The details of this curse are unknown.

Destruction
In the summer of 1996, Dumbledore investigated Tom Riddle's fascination of horcuxes, and found the ring in the Gaunt shack. He was able to destroy the ring, but severely damaged his own right hand while doing so.