The law is on your side and it does not help
California requires loads to be secured and covered so nothing escapes the vehicle. A gravel truck shedding stones is in violation.
The problem is entirely practical: proving that a specific stone came from a specific truck requires evidence you do not have at 65 mph. Insurers and courts want more than "it must have been that one," and most claims die there.
If you are going to try
You need the plate, the company name from the door, the time, the location and the direction of travel. Photograph the truck if it is safe and legal to do so — pull over, do not chase.
Some companies carry a fund for exactly this and will pay a claim if you can produce those details promptly. Many will not. Report the vehicle to CHP either way; a pattern of reports about one operator does eventually produce enforcement.
The prevention that works
Distance. Stone velocity falls off fast, and most of the damage comes from being close enough that the stone still carries its full speed.
Give any truck carrying aggregate, any trailer with an open bed, and anything with visible debris in the tailgate a wide berth — well beyond normal following distance — and change lanes rather than sitting behind it. On Highway 50, Highway 65 and Interstate 80 through the region, this is a habit worth building.
And repair it fast
Whatever you do about the truck, treat the chip as urgent. In this climate a repairable chip becomes an unrepairable crack in weeks. That part is under your control.
Need this done rather than explained? There is no shop to drive to. We come to wherever your car already is — your driveway, your workplace car park, an apartment lot, or the side of the road it stopped on — anywhere in greater Sacramento, with no call-out fee and no mileage charge. Give us the year, make and model and you will have a firm price before the call ends.