The great jewel discovered beneath the roots of Erebor by Thráin I soon after the establishment of the Dwarf-kingdom there, and prized by his descendants as the Heart of the Mountain. The Dwarves used all their skill to work the gem into a shimmering multi-faceted jewel that not only 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.' (The Hobbit 13, Not at Home). It was a heavy gem, small enough for Bilbo to hold in one hand, yet not so small that he could close his own small hand around it.
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 Smaug sacked Erebor, 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. Later, when the Lake-men and Wood-elves came to demand their own shares of Smaug's treasure from Thorin, Bilbo delivered the Arkenstone to them to bargain with. In the ensuing Battle of Five Armies, though, all enmities were forgotten, and 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.