What does the company find in Smaug's lair? what is arkenstone?

They find a huge dragon treasure. He follows the treacherous course into the heart of the cave and though he is sure he is in danger, he is attracted by a red glow that compels him to approach. This is the glow of Smaug. Bilbo manages to steal a cup and hurriedly exits but Smaug awakens and begins to rage. Bilbo has really become an expert burglar by this point and he has claimed for himself the one artifact that Thorin finds most valuable‹the Arkenstone, the Heart of the Mountain.