"Mad to you, perhaps, but not to Voldemort, I hope you will understand in due course exactly what those objects meant to him, Harry, but you must admit that it is not difficult to imagine that he saw the locket, at least, as rightfully his"
-- Albus Dumbledore (HBP20)
A heavy gold locket carrying Salazar Slytherin’s mark (an ornate serpentine S). Inherited by Merope Gaunt Riddle, pawned during her pregnancy to Borgin and Burkes, purchased by Hepzibah Smith, and subsequently stolen by Tom Riddle upon her murder (HBP20). It was made into a Horcrux by Voldemort.
At some point, the locket was removed from its hiding place by Regulus Black and Kreacher, who together substituted another locket for the original and left a note for Voldemort. Kreacher subsequently tried and failed repeatedly to destroy the locket as Regulus Black had ordered him to. It was later found at number twelve, Grimmauld Place during the summer before Harry’s fifth year (OP6)
Dumbledore & Harry try to acquire it, although they only find the decoy left by Regulus Black (HBP26). Kreacher reveals Mundungus stole it, who reveals that Umbridge confiscated it (DH11). Harry, Ron & Hermione steal it from her (DH13) and Ron eventually destroys it using Gryffindor's sword (DH19).