While the dwarves panic about being seemingly trapped inside the Mountain, Bilbo suggests that the only way out is down-into Smaug's lair. Not knowing whether Smaug is present, the company nevertheless goes down into the dragon's cave. Among the treasures in Smaug's hoard Bilbo finds the Arkenstone-the large, precious jewel that Thorin especially wishes to recover. He puts it in his pocket, hoping that he can choose it as his share of the reward, although he suspects the dwarves did not intend for him to have the choicest of the treasure, "the Heart of the Mountain" itself. Thorin, in fact, searches the cave for the Arkenstone, but does not find it. He does, however, give Bilbo a princely coat of dwarf chain mail. The dwarves outfit themselves in valuable mail as well before leaving Smaug's lair, still wondering where the "old worm" himself is.
Tolkien employs irony as he shows readers how Bilbo, who was hired by the dwarves to be a burglar, in turn "burgles" them. Readers should note that, in taking the Arkenstone, Bilbo seems to be motivated by nothing more than, in the narrator's phrase, "the bewitchment of the hoard." Shortly, however, he will be able to "redeem" or "justify" his action (just as he has justified his action in taking the Ring, albeit at first unknowingly, from Gollum, by using it to save himself and his friends). The subsequent events, of course, only raise the old question of whether ends-including unintentional ends-justify means.
The Hobbit: Novel Summary: Chapter 13