SAN CLEMENTE, Calif. (AP) -- Authorities in Southern California are investigating what appears to be the spontaneous combustion of rocks scooped from a San Clemente Beach that left a woman with third-degree burns.

Orange County Fire Authority Capt. Marc Stone tells City News Service the woman's children collected the rocks Saturday and she tucked them into her pocket.

Stone says at some point the rocks caught fire, burning the woman's right thigh and knee.

Her husband also suffered second-degree burns to his hand trying to put out the flames.

Authorities didn't provide any details about what would have caused the rocks to ignite. Stone says it should take health authorities a week to complete their investigation.