The powerful jewel sought by Thorin Oakenshield in J.R.R. Tolkien’s The Hobbit is the Arkenstone. Also called the King’s Jewel, it is a large, radiant gem treasured by the royal line of the dwarves of Erebor.
Smaug keeps the Arkenstone among the hoard beneath the Lonely Mountain. Bilbo finds it during his secret exploration and hides it, later giving it to Bard and the Elvenking Thranduil in an attempt to prevent a battle between the assembled forces and Thorin’s dwarves.
The Arkenstone is not one of Tolkien’s Silmarils. The Silmarils are three much older jewels central to the First Age stories, while the Arkenstone belongs to the later history of Erebor. It is also distinct from the Elfstone, a name associated with Aragorn’s green jewel in The Lord of the Rings.