Bilbo never intended to keep the Arkenstone. He only took it because he saw the feverish greed that overtook the Dwarves when the saw their long lost treasure. Knowing how badly Thorin coveted the Arkenstone, Bilbo kept it and later used it to bargain for his share of the treasure (which he intended to share with Bard and Thranduil).
What it revealed was that deep at heart, Bilbo was a good person and only wanted to do what was right. It was why Gandalf was drawn to him in the first place.
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He is hoping that the desire for the Arkenstone would force Thorin to negotiate with the Men and Elves instead of fighting with them.Bilbo believes that if the men and elves have the Arkenstone, which Thorin values even more than gold, they may be able to convince the dwarves to bargain with them. If an agreement can be reached, it will spare a good deal of bloodshed.Also, Bilbo is getting quite tired of being trapped in the mountain, and wants to be somewhere with green grass underfoot and better meals.
Well, as he justifies it to himself, he was entitled to a one fourteenth share of the treasure and which fourteenth was not specified. Though he was pretty sure that the Arkenstone would not be a part that he would get.
Smaug tells Bilbo not to trust dwarves:In return for the excellent meal [of the Company's ponies] I will give you one piece of advice for your good: don't have more to do with dwarves than you can help!Smaug-The Hobbit: "Inside Information"
Gandalf's presence with the group camped around the dwarven stronghold delighted Bilbo.gandalf
Gollum was first known as smeagul but once he found the ring and killed his brother he was then known as Gollum (he got this name when the ring took controlled of him and he started making the chocking noise sounding like the word Gollum). He had the one ring for hundreds of years giving him unnatural long life. He became fixed on the ring and would kill anyone who would take it like he tried to do to Bilbo once he noticed he had lost it. Gollum would not give the ring up easily and tried to take it from Frodo in The Lord Of The Rings. Bilbo had it for much shorter time, so part of his own mind was still in control and was not as obbsessed with the ring as Gollum, and was able to find the will to give it up and pass it on to his adopted heir, Frodo Baggins.