The Arkenstone of Thrain, also known as the Heart of the Mountain and the King's Jewel, was a wondrous gem sought by Thorin Oakenshield which had been discovered beneath the Lonely Mountain by Thorin's grandfather Thror, and then shaped by the Dwarves. The Arkenstone had been the family heirloom of Durin's folk, but was lost when the dragon Smaug captured the mountain from the Dwarves.