Arkenstone

Warning: Wikipedia contains spoilers

The Arkenstone was a wondrous gem sought by Thorin in The Hobbit. When Bilbo Baggins found it on Smaug's golden bed deep inside the Lonely Mountain, he pocketed it. When the Dwarves refused to reward Bard for killing Smaug, Bilbo snuck out of the Dwarves' fort inside the Mountain, and gave him the Arkenstone; Bard then tried to trade it for Bilbo's fourteenth share of Smaug's hoard. The dispute was interrupted by an evil army arriving from the Grey Mountains, a battle ensued, and Thorin was killed. The Arkenstone was placed on Thorin's chest when he was buried, as was his great sword Orcrist.