The Arkenstone was a gem, the object most prized by Thorin Oakenshield of all the treasures of the Lonely Mountain. Such did he consider its value that he was willing to trade 1/14th of all the gold and silver of Smaug's hoard for it.
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, the man 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. Then an evil army arriving from the Grey Mountains interrupted the dispute, the Battle of the Five Armies ensued, and Thorin was killed. The Arkenstone was placed upon Thorin's chest within his tomb deep under the Lonely Mountain, and so was returned to the earth at last.