The Arkenstone

Found in the deep mines of Erebor in the earliest days of the Dwarf-hold’s founding, this pale jewel – known as "the Heart of the Mountain" – is the greatest treasure ever unearthed by the Dwarves. The skill of the Dwarf-smiths transformed the already beautiful stone into a fist-sized orb, with a thousand flawless facets; it shines with an inner luminescence, but when the merest sliver of outer light falls upon it, it gleams like water in the sunlight, shot through with the colours of the rainbow.
In the centuries after its discovery, the Arkenstone became an heirloom of the Kings of Durin’s folk. Throin’s son, Thorin, carried it away into the Grey Mountains where it remained until King Thror brought it back to the Great Hall of Throin. When the Dragon Smaug sacked Erebor, the Arkenstone was thought lost, lying among Smaug’s treasure trove in the bottommost cellar until Bilbo Baggins discovered it and gave it to Bard. After the Battle of Five Armies, enmity was set aside between the leaders of the Dwarves, Elves and Men, and Bard of Dale placed the Heart of the Mountain in Thorin’s tomb. Nearly a thousand years after its discovery, the Arkenstone is buried once more beneath the Lonely Mountain.