Mwah~! I'm really glad someone else has had the same idea floating around in her head about the Arkenstone being a Silmaril. Though Thranduil prolly should've recognized it for what it was, having been in Doriath with King Thingol and Melian. The death of King Thingol at the hands of the dwarves, over the Nauglamir, is likely the start of Thranduil's distrustfulness of dwarves, anyway.