Well, if anyone would care to read it who hasn't already, I've actually written rather an extensive analytical on this subject. You can have a look at it
here. Some discussion follows on this thread (I think it's about nine or so pages back in this forum).
I wrote the essay from the point of view that yes, the Arkenstone was a Silmaril. However one cannot really decide this to be true. It does seem unlikely, but it is quite possible, I think, that it may have been. Of course, the similarities may just be down to the fact that both gems were created in the brain of the same author.
[ July 06, 2003: Message edited by: Gwaihir the Windlord ]