The Arkenstone was a great Jewel found in Erebor by Thráin I. The dwarves named it the Heart of the Mountain and treasured it. It was taken by Smaug the Dragon and found by Bilbo Baggins after Smaug was slain by Bard. Knowing how much Thorin wanted the jewel, Bilbo used it as a ransom just before the Battle of Five Armies.
