I hit a truck tire recap in my M3 some years ago. The insurance adjuster asked me if the tread was moving when I hit it on the highway, or stationary. It was moving, and as a result, the damage was covered under comprehensive (no deductible). If it would have been stationary, it would hav been covered under collision (deductible).
Obviously this rule does not cover moving/statioary vehicles, and I'm not sure if it covers rocks, but it could not hurt to check.
Anybody an insurance pro out there?