The rocks contain various colors - some green, some orange. There were seven of them altogether, collected by a San Clemente family during a fun morning at Upper Trestles on San Onofre State Beach last Saturday.
"The daughters were playing with the rocks, and she took them away, and I guess basically, they combusted in her pocket due to friction," said neighbor Jacob.
Orange County Fire Authority Capt. Marc Stone said there were actual flames coming off the woman's cargo shorts.
"We found that a 43-year-old female did have severe burns to her leg, second- and third-degree burns," Stone said.
The incident remains under investigation. The county's health care agency says they're waiting for further test results on the rocks, but they believe two of them contained a phosphorus substance. The other five became cross-contaminated by being together in the victim's pocket when the reaction happened.
How phosphorus rocks could have ended up on the beach is unclear, but county officials say environmental health workers visited the area and did not find a public safety problem - only adding to the mystery of how it could happen.
"I talked with the paramedic today. He's been on numerous calls for 27 years. He's never seen anything like it," Stone said.
Authorities say the woman's husband also suffered burns to his hands as he was trying to help his wife.