I have a friend who keeps a large gear, weighs about 10 pounds, on his piano. A reminder that everyone in the minivan survived when it was thrown through the winshield. So much for harmless rocks. As harmless as dropping concrete blocks off an overpass.
My father had a large rock dropped from an overpass through the windshield of his 18 wheeler. Luckily, it hit on the passenger side, which was empty at the time.