What is the Arkenstone that Thorin recognizes?

From the text:

So Bilbo guessed from Thorin's description; but indeed there could not be two such gems, even in so marvellous a hoard, even in all the world. Ever as he climbed, the same white gleam had shone before him and drawn his feet towards Slowly it grew to a little globe of pallid light. Now as came near, it was tinged with a flickering sparkle of man colours at the surface, reflected and splintered from the wavering light...Ch 13