- "That stone of all the treasure I name unto myself, and I will be avenged on anyone who finds it and withholds it."
- ― Thorin
The Arkenstone was a great jewel discovered beneath the roots of Lonely Mountain and prized by his descendants as the "Heart of the Mountain".
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 led a band of Dwarves to recover their ancient city, their companion Bilbo Baggins discovered the Arkenstone, and kept it for himself. While exploring the treasure, Thorin looked forward to find it, but despite seeing his anxiety, Bilbo decided to keep it secret. Later, when the Lake-men and Wood-elves came to demand their own shares of Smaug's treasure from Thorin, Bilbo noticed Thorin's nonchalant stance. To aid the two factions, he secretly entered their camps and delivered the Arkenstone to their leaders, Bard and Thranduil to bargain with. Indeed, the next day Thorin was demanded the treasure in return for the Arkenstone.
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.
The Dwarves worked the stone into a multifaceted jewel. It 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 but small enough for Bilbo Baggins to hold in one hand, yet not so small that he could close his own small hand around it.
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.
 Other Versions of the Legendarium
In the writing of The Hobbit J.R.R. Tolkien halted after the capture of the Dwarves by the Wood-elves and sketched out the continuation of the story in plot notes. In these notes Bilbo stole a "gem", which became a "marvelous gem", and finally the "Jem [sic] of Girion" which the king of Dale had given to the Dwarves for the arming of his son. As noted by John D. Rateliff, the Gem of Girion was to be offered by the Dwarves to Bilbo as his share of the payment, thus solving the problem of how to transport Bilbo's one-fourteenth share back to Bag End.
Tolkien originally had the notion that Bilbo would kill Smaug. As his story progressed Tolkien decided that this strained credibility and so he introduced Bard as the dragon slayer and made him the heir of Girion. This led to a series of re-conceptualizations since now the Lake-men and the Elves were no longer mere opportunistic scavengers; there was one within their ranks with a legitimate claim to part of the treasure. The Gem of Girion became the Arkenstone and the one part of the hoard that Thorin would insist upon keeping. The elevation of Girion's gem led to the invention of the Necklace of Girion as the payment of Girion to the Dwarves for the arming of his son.
- ↑ 1.0 1.1 J.R.R. Tolkien, The Hobbit, "A Thief in the Night"
- ↑ 2.0 2.1 J.R.R. Tolkien, The Hobbit, "Not at Home"
- ↑ J.R.R. Tolkien, Christopher Tolkien (ed.), The Shaping of Middle-earth, The Earliest Annals of Valinor
- ↑ "New evidence for the Arkenstone-Silmaril case" , Forum.barrowdowns.com (accessed 09 June 2013)
- ↑ "RE: Eorclanstanas (was Re: For JDR: Query on Hobbit geography)" dated 02 January 2007, Mailing list for the Mythopoeic Society (accessed 09 June 2013)
- ↑ J.R.R. Tolkien, John D. Rateliff (ed.), The History of The Hobbit, Mr. Baggins, The Second Phase, "Plot Notes B", p. 364
- ↑ J.R.R. Tolkien, John D. Rateliff (ed.), The History of The Hobbit, Mr. Baggins, The Second Phase, "Plot Notes B", (vii) The Gem of Girion, p. 373
- ↑ J.R.R. Tolkien, John D. Rateliff (ed.), The History of The Hobbit, Return to Bag-End, The Second Phase, "While the Dragon's Away...", (ii) The Arkenstone as Silmaril, p. 603