Rich wrote:
I forgot to mention that I have a two year old boy who has a habit of
picking up rocks and licking them, or putting them in his mouth... when I'm
not looking of course. I catch him doing this every time I turn my back on
him. So this probably isn't a good idea, is it? Or, could it be sprayed off
the gravel and still be effective? I just don't know if this is a good idea
after all.

Given the heat and the reflective surface of the rock, vinegar could do
the trick. Might also discourage rock tasting.