Lyn Hiner's children collected seven colorful rocks at San Onofre State Beach on Saturday. She stashed them in her pocket, and hours later, they somehow ignited.
Hiner is recovering after undergoing surgery at the Grossman Burn Center in Santa Ana.
"All of a sudden, something hot on my leg just sort of started to bother me, and so I started to think it was a bug bite, and I started slapping it, and the next thing I know, my shorts are on fire," she said on "Good Morning America."
Hiner dropped to the floor of her kitchen and rolled in an effort to douse the flames. Her husband Robert burned his hand while trying to help her.
"Our first response was just trying to pat it out. We didn't know what it was. It was just this bright, intense flame," he said. "It wasn't going out. Eventually, we just tore her shorts off and got them off her."
Orange County fire officials say the flames ignited the floor of the family's home.
Her doctor says this case was a first for her.
Authorities believe at least two of the rocks were coated in phosphorus, but how the chemical got there remains a mystery. Phosphorus is a highly flammable substance that can burn like a road flare.
The beach is located near the San Onofre Nuclear Power Plant, and it's also close to Camp Pendleton. Officials with the Marine base say they are cooperating with investigators, but so far, there's no evidence that material from training exercises was involved.
The Hiners expressed gratitude to everyone that helped them because they realize it could have been much worse.