Copied this from here: http://tolkiengateway.net/wiki/Misconceptions#The_Arkenstone_was_a_Silmaril
>"The Arkenstone was a Silmaril, probably the one thrown into a fiery pit by Maedhros, and found its way (geologically?) to the north, to be rediscovered by the Khazad of Erebor. Tolkien wrote that the two lost Silmarils would remain lost until the end of Arda. However, in a partial translation of early Silmarillion texts into Old English Tolkien used the etymologically related term 'Eorclanstanas' ('holy stones') to translate 'Silmarils' - suggesting that he may have borrowed the name and other concepts from the Silmarils in describing the Arkenstone.
A Silmaril is a gemstone hallowed by Varda which would not suffer the touch of mortal or evil hands.[1] It only allowed Beren to handle it, but Beren also lost the hand that held it. Many mortals handled the Arkenstone without a consequence, and they did not all have good intent. It would have burned them and possibly burned Smaug if it were truly a Silmaril. As attractive as this theory is, because it would have made the Arkenstone such a poignant element in The Hobbit, it can only be, at best, Silmaril-like or Silmaril-inspired. The latter being the most likely case. It is not a Silmaril."
Whether Tolkien really verified it himself or not it still stands that the Silmarils themselves could not be touched nor handled by mortals. I personally would have loved the idea of it being worked into the story. Even then though, since the Silmarillion itself is more a chronicle of myths and epics what could be considered fact or fiction to the reality of Middle Earth is up for debate. If you take the Silmarillion as the ultimate final word then by definition the Arkenstone would likely just be it's own special stone