The Arkenstone was a jewel discovered beneath the roots of Erebor by Thráin I, and prized by his descendants as the Heart of the Mountain. The Dwarves worked it into a shimmering multi-faceted jewel that shone by its own light, but when light shone upon it, it '...changed it into ten thousand sparks of white radiance shot with glints of the rainbow.' (The Hobbit)
In the centuries following its discovery, it became an heirloom to the Kings of Durin's Folk. It was carried away by Thráin's son into the Grey Mountains, and was eventually brought back to the Great Hall of Thráin under the Mountain by his descendant Thrór.
When the dragon Smaug took Erebor (T.A. 2770), the Arkenstone was lost, finding a new home among Smaug's booty in the halls of Erebor. Many years later, Thorin Oakenshield led a band of Dwarves to recover their ancient city and stolen treasures; Bilbo Baggins discovered the Arkenstone (T.A. 2941), and took it, knowing how valuable it was to Thorin. When the Lake-men and Wood-elves demanded their shares of Smaug's treasure, Bilbo offered the Arkenstone to bargain with. Following the Battle of Five Armies, Bard the Bowman of Dale placed the Heart of the Mountain on the breast of Thorin in his tomb beneath Erebor; once more, the Arkenstone was buried beneath the Lonely Mountain.
|Thorin's Burial (Alan Lee)|
Tolkien took the name from the Old English earcanstān (also spelled eorcanstān, eorcnanstān) or the Old Norse jarknasteinn, meaning "precious stone".