There was an interesting question about the maximum speed of a character, with an even more interesting answer that a properly optimized character can break the speed of light. (So, isn't general relativity present in D&D, and is there no time dilation, etc.?)
If someone can reach such ludicrous speeds, is would be certainly possible to also throw objects with nearly the speed of light. This would lead to an interesting phenomena, as you could turn any piece of rock into a nuclear warhead.
Would it then be according to the rules, that any character being able to reach such accelerations and speeds, could destroy huge cities just by throwing pebbles at it with 0.9 c?