The BBC reports that a German woman was walking along the Elbe river near Hamburg when she happened upon a beautiful rock. Thinking it was a piece of amber, she put it in her jacket pocket. She laid her pocket on a bench, turned around, and when she turned back her pocket and the bench were aflame.
The material the woman had picked up wasn't a rock. It was a piece of white phosphorous that had probably been laying in the river, all damp and neutralized since it fell there during World War II while in an incendiary device. White phosphorous basically bursts into flames as soon as it's dry, and burns as high as 1,300 C. It's also extremely hard to put out as long as it's surrounded by oxygen.
In this YouTube video, an at-home scientist shows exactly how quickly the material lights up and the difficulties of stopping the flames. He even burns a piece of meat so you can guess what it would look like if it were touching your flesh: