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:
Basically, this woman is really lucky she didn't have her jacket on.
The mix up is common enough that authorities have warned people collecting amber to store their finds in tins until they have time to lay them outside somewhere and see if they spontaneously turn into fire. Hamburg is one of many German cities that were heavily targeted by Allied forces, and old weapons that haven't been activated are frequently found.
According to the Telegraph, a 1.8 ton bomb dropped by Britain’s Royal Air Force was found in December of 2016, in Augsburg. Over 50,000 people had to evacuate the city on Christmas Day as authorities launched a disposal operation.
An estimated quarter million more explosive weapons are likely to still be underground, and tens of thousands are diffused each year.
Firefighters came to put out the blaze, and eventually succeeded, but it's a scary reminder of violence in our past that still haunts us—and also to not pick up trash on the ground and put it in your pocket.