Answer:
A lost crown that once belonged to Helena Ravenclaw. Her daughter, the Grey Lady, stole it and took it to Albania where is was considered lost. Lord Voldemort discovered it, turned it into a Horcrux and hid it inside the Room of Requirement at Hogwarts.
He hid it inside the room when it was turned into a place to hide things, Harry Potter hid the Half-Blood Prince potion book in there the year before so he knew what it was. Voldemort thought nobody else would ever come across the room.