[Mithadan]

Let me clarify. It cannot be doubted that Bilbo's decision to turn the Arkenstone over to Bard was motivated by good intentions. But the decision to take the Arkenstone and later conceal it after Thorin began searching for it is a bit out of character for Bilbo. Could this be the Ring's effect (from the perspective of a coherent story)? Perhaps. He thinks to himself that Thorin's offer that he could pick and choose his share might not extend to the jewel. Thorin later names the Arkenstone to himself, but Bilbo does not come forward.

But candidly, the language used by Tolkien suggests another explanation. "Bilbo's arm went to it drawn by its enchantment." One can also refer to the poem The Hoard found in Adventures of Tom Bombadil where he discusses the power and bewilderment of treasure. The original title of this poem (it was first published in 1923 in a Leeds University magazine) Iumonna Gold Galdre Bewunden, meaning Gold of Men Enmeshed In Enchantment (See Annotated Hobbit, p. 288). This is a recurring theme in Tolkien's works, the love of treasure as a motivator towards either evil or bad decisions. I tend to favor this explanation of the Arkenstone incident.

However you slice it, Bilbo's concealment of the Ring from Gandalf and his taking and retention of the Arkenstone is not typical behavior. The cause? It could just as easily be the allure and glamor of fantastic treasure as the emerging effects of the Ring.

By the way, Gandalf's "more than meets the eye" comment after Bilbo explains his escape from Goblin Town (omitting the Ring) and the long stare that Gandalf gives to Bilbo as if he doubts some of the tale is in the original, pre-LoTR edition for those keeping score on the ring vs. Ring issue.