He probably didn't know what it was. He was just really mad, and that was the only thing availible, so he picked it up and chucked it at Gandalf. I bet after Saruman got after him he really wished he hadn't thrown it! But I think it was just that he didn't have a clue as to what it was.
Arwen
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Will Turner: "This is either madness or brilliance."
Jack Sparrow: "It's remarkable how often those two traits coincide." ~ Pirates of the Caribbean
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