A woman in Orange County, California discovered that rock collecting can be a dangerous hobby, when the rocks she'd collected from the beach earlier that day suddenly ignited in her pocket!
Here's the scoop:
Orange County Fire Authority officials tell the newspaper that the woman collected the rocks on a nearby beach, returned home and "was standing in her kitchen ... when the pocket of her cargo shorts caught fire."
"I talked to the paramedic who treated her, and in his 27 years in responding to calls near the beach, he's never seen this," Fire Authority Capt. Marc Stone told the Register. "The rocks were still smoking when firefighters took them to the hospital."
Now, they're being tested. It's possible, Stone said, that phosphorus in the stones may have caused the combustion.
And they say hobbies like rock and shell collecting are a safe and relaxing activity.
This phenomenon is will known in Germany, there is even a sign on the island of Usedom that warns people to pay attention what kinds of rocks they gather (German): http://www.stefannehring.de/phosphor-usedom.htm
In a report on TV, a doctor was quoted as saying that the burns are very nasty and most follow the same pattern: upper leg (because people put the stones into their pockets) and one hand (when people try to get the then ignited phosphorus out of their pocket again.
Simple safeguard: Put amber-like rocks into a bucket, basket or even your handbag - everywhere but in your pockets! When dried and still not burning, it's not phosphorus.
More on white phosphorus: http://en.wikipedia.org/wiki/White_phosphorus