SAN CLEMENTE, Calif. â€"  A woman suffered third-degree burns after what appeared to be the spontaneous combustion of rocks that had been scooped up from a Southern California beach, a fire official said.

The 43-year-old woman's children collected the rocks Saturday at San Clemente Beach, and she tucked them into the pocket of her shorts, according to the Orange County Register. While the woman was standing in her kitchen Saturday after returning from the beach, the rocks somehow caught fire, burning the woman's right thigh and knee, said Capt. Marc Stone of the Orange County Fire Authority.

The woman tried to stop, drop and roll but couldn't extinguish the flames, Stone said.

So what caused the mysterious fire? 

"There are three kinds of answers," Cory Powell, editor of Discover Magazine, told Fox News. "Sometimes people just do stupid things that are hard to detect. It's possible the woman had a lighter or a can of sterno in her pocket, and a spark from the rock ignited it." 

"It could be something as simple as that," he said. Other possibilities include an unexploded shell from a nearby military base -- or the potential that the rocks themselves contained phosphorous, a natural element that can spontaneously combust.

"It would be really weird that those rocks are lying around on the beach, not touching anything -- but it's possible," he said. 

The rocks are small, smooth, and orange and green in color. They eventually fell from the shorts onto the wooden floor where they continued to burn and fill the house with smoke, Stone said.

The woman's 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, saying they're undergoing testing to see what happens.

"There is phosphorous that naturally occurs on the sand at the beach, but no one has ever heard of pants catching fire," Stone told the Register.

Stone said it could take weeks to complete the investigation.

The Associated Press contributed to this report.