Arkenstone???

I will say no it is not a silmaril. The hobbit was written just after tolkien finished writting his first sketches of the Silmarillion. He had already come up with the tale of the silmarils and since the Hobbit was for his family they would have known what the silmarils were. Also as has been pointed out earlier there were oly three and they are accounted for as to where they are. The silmarils would burn through cloth and blister skin. Yet Bilbo wrapped it in his cloak for days. Also Thranduil would have recognized it as a silmaril when Bilbo gave it to him. If it truly were a silmaril Thranduil would have left with just that gem and would not have cared for any of the other treasure-yet he stayed to continue the war for the treasure and used it as a bartering tool.