Arkenstone

The Arkenstone (or Heart of the Mountain) of Thrain was a wondrous gem sought by Thorin Oakenshield in J. R. R. Tolkien's The Hobbit. It was discovered beneath Erebor (the Lonely Mountain) by Thorin's ancestor Thrain and shaped by the Dwarves. The Arkenstone became the family heirloom of Durin's folk, but was lost when the dragon Smaug captured the mountain from the Dwarves.

When Bilbo Baggins found it on Smaug's golden bed deep inside the Lonely Mountain, he pocketed it, having learned how much Thorin valued it. While the Dwarves with Thorin sorted the treasure, Thorin sought only the Arkenstone, unaware that Bilbo was hiding it in his pillow. When the Dwarves refused to share any of the treasure with King Thranduil and Bard, who had killed Smaug the Magnificent, Bilbo crept out of the Dwarves' fort inside the Mountain, and gave them the Arkenstone; Bard, Thranduil, and Gandalf 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, the Battle of Five Armies ensued, and Thorin was killed. The Arkenstone, and the great sword Orcrist, was placed on Thorin's chest when he was buried deep under Erebor.