He wasn't sure, the pale glow mesmerized him until he picked it up.
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He stole it from Smaug, to prove to the dwarves that he was an excellent burglar.
The Arkenstone of Thrain.
He had possession of the Arkenstone.
Bilbo hides the Arkenstone from the dwarves because he knew each one was greedy for the Arkenstone and they might kill each other or harm others for it.
thorins is much powerfull
There are two ways that he is selfish: 1. He refuses to share the Ring 2. He refuses to share the Arkenstone