The pilot stated that he was landing off-airport to pick up some parts at a parts house. The landing was made on a driveway, which was 500 feet long and 20 feet wide. During the landing, the aircraft 'snagged a rock' and nosed over, causing substantial damage. The pilot stated that turbulence moved the aircraft to the left side of the driveway, where it impacted the rock. Also, he said that heavy braking and impacting the rock caused the aircraft to go over on its back. The pilot noted that the accident could have been avoided 'by not landing here.'