[Theron Bugtussle]

As Kalimac said, Saruman kept the palantir secret. Even from Gandalf, when he visited seeking information on the ring and was imprisoned on the pinnacle of Orthanc.

When the palantir was thrown, that was the first that Gandalf knew that Saruman had one. He didn't seem to think that any other even survived. Until he was mulling it over with Pippin on the ride to Minas Tirith. Then he realized that Sauron must have had one as well. Gandalf was able to tie up many loose ends with that deduction.

(Certainly PJ screwed that up in the movies by having Saruman show Gandalf the palantir before the Wizards' Duel.)

So I think this is how it happened to be the item that Grima threw:

Gandalf called to the tower from the entrance stairs. Grima came and was told to fetch his master. Saruman was interrupted by Grima in his "study" probably using the palantir. Though, I doubt he let Grima see the stone at that time.

Saruman was surprised/shocked to hear of any visitors, let alone these particular ones (Gandalf & co.), so in his distress [img]smilies/eek.gif[/img] and rush he left his study unlocked. Saruman went to the window balcony, on a lower floor than where the palantir was.

That distress and lack of attention allowed Grima to enter the 'study' (or wherever the palantir was), overhear the whole conversation, get mad [img]smilies/mad.gif[/img] , find the palantir, and throw it. He would have known nothing of its usefulness, but would have discerned that it was a thing of some importance to his master--because of its prominence in the location where he found it. And have seen it as a useful missile. [img]smilies/biggrin.gif[/img]

I fail to see the claim of its being basketball or bowling ball size...where is that from? Gandalf took it from Pippin and hid it in the folds of his cloak. Can you imagine "hiding" a stone of that size in your cloak? You would need a sling of some kind. [img]smilies/eek.gif[/img]