The silmarils were made from a new unique material. My guess is that the arkenstone was some of that material, but not a silmaril. It does have the power to capture the light from the undying trees, as it glows on its own, but it is not a complete silmaril, as all of them were lost in different places.
John Doyle
Alaccountu Sulemana Frinjei
Amber Elizabeth Lewis
Ena Pericas
Thet Myat
Susan Jordan