Legal Question in Personal Injury in California

Re: What proof do I have?

If the rock came up from the ground by the tires, then, the company will argue that it was an "act of god" and they are not liable. Which, is probably correct. However, if the rock actually came from the truck, then you have a case if you can prove it in small claims. Go to the nearest courthouse to get small claims forms and information on filing. Good luck and thanks for inquiring.

Re: What proof do I have?

You can offer your own testimony to the small claims judge as proof. If the truck was carrying a load of gravel then you stand a good chance of winning.

Non-lawyers often don't realize that testimony is a form of evidence and that cases can be won with no physical evidence, so long as the testimony is convincing.

Of course, you will also want to bring in pictures of the damage and invoices for the repair (or estimates if the repair has not been done yet), if possible.