It might have been white phosphorus, which is used as ammunition . When wet, the untrained eye can mistake it as amber. When in contact with air, it ignites and burns at 1300C (2372 F). As long as the rock has a protective layer of water, it is not dangerous, but as soon as it starts to dry...
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
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