- "That stone of all the treasure I name unto myself, and I will be avenged on anyone who finds it and withholds it."
- ― Thorin Oakenshield
The Arkenstone was a great jewel discovered beneath the roots of Lonely Mountain by Thráin I soon after the establishment of the dwarf-kingdom there, and prized by his descendants as the "Heart of the Mountain".
The Dwarves used all their skill to work the gem into a shimmering multi-faceted jewel that not only shone by its own pale light, but when light fell upon it, the stone "...changed it into ten thousand sparks of white radiance shot with glints of the rainbow". It was a heavy gem, small enough for Bilbo Baggins to hold in one hand, yet not so small that he could close his own small hand around it.
In the centuries after its discovery, the Arkenstone became an heirloom of the Kings of Durin's folk. It was carried away into the Grey Mountains by Thráin's son, and in time brought back to the Great Hall of Thráin under the Mountain by his descendant Thrór. When the Dragon Smaug sacked Lonely Mountain, the Arkenstone was lost to the Dwarves of Durin's Folk — it lay among Smaug's booty in the halls of Erebor.
Many years later, when Thorin Oakenshield led a band of Dwarves to recover their ancient city, their companion Bilbo Baggins discovered the Arkenstone, and kept it for himself. Later, when the Lake-men and Wood-elves came to demand their own shares of Smaug's treasure from Thorin, Bilbo delivered the Arkenstone to them to bargain with. In the ensuing Battle of Five Armies, though, all enmities were forgotten, and afterwards Bard of Dale placed the Heart of the Mountain on the breast of Thorin in his tomb beneath Erebor. So, nearly a thousand years after its discovery by Thráin I, the Arkenstone was buried once more in the depths beneath the Lonely Mountain.
Arkenstone means roughly "precious stone", although the element *arken does not exist in modern English. Arkenstone is a modernization of an ancient word which appears in the Edda as jarknasteinn and in Old English as eorclanstán.
Note that Tolkien used the word eorclanstánas to refer to the Silmarilli in Old English texts by Eriol. Some fans explore the possibility that the Arkenstone was one of the Silmarilli, specifically the one Maedhros threw in the chasm, until supposedly found by the Dwarves of Erebor.