Friends say Lyn Hiner is recovering after surgery. The 43-year-old suffered third-degree burns to her right leg.
Firefighters were called to the Hiner's home in San Clemente Saturday afternoon, about four hours after she first put the rocks in her pocket.
"The husband was outside with a garden hose actually trying to cool her leg down. The fire that occurred in her shorts from these rocks actually set the smoke alarms off in the house," said Orange County Fire Authority Capt. Marc Stone.
There were seven rocks altogether, collected by Hiner's daughters on Upper Trestles at San Onofre State Beach.
"They were just small, little - they looked normal, basically orange and white rocks," said Jacob, a neighbor.
The Orange County Health Care Agency is investigating. Officials say they're waiting for further test results, but they believe two of the rocks contained a phosphorus substance.
One geologist, not related to the investigation, says he does not believe Mother Nature left it behind.
"Somebody has put orange material on top of that rock, and that's what's flammable," said geologist Dr. Pat Abbott. "Something that may be from fireworks or flares or explosives or some human-made material put on that rock is what caught on fire."
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.
"It doesn't worry me, but it's peculiar," said Lauren Griffin of San Clemente.
Environmental health officials say they visited the beach where the rocks were collected, and they did not find any public health hazard. Testing of the rocks is expected to take a couple weeks.