Why did he throw that?

[Morgoth the Great]

i'm sorry, but i have to say that he had no clue what it was. i doubt that Saruman would of told him, and he would not of been so foolish enough to leave is unguarded without a good reason. Grima probably saw that Saruman was not doing well in his argument, and he most likely also wanted to pay back the Rohirrim for banishing him. The stone, being a heavy round object with potential to do great damage was probably a good thing to throw down, as if it had struck anyone, it could of done serious damage. Grima probably didn't see it as a Palantir, more as the best thing to throw.