A German woman narrowly escaped injury after picking up an object she believed to be amber but which then spontaneously combusted.
She had plucked the small object from wet sand by the Elbe river near Hamburg and put it in a pocket of her jacket, which she laid on a bench.
Bystanders soon alerted the 41-year-old to the fact her jacket was ablaze.
The stone was actually white phosphorus, which had reacted with the air as it dried.
Police say the two are easily confused.
They are warning local beachcombers to collect amber in tins, saying pieces of phosphorus dropped in incendiary bombs by the Allies in World War Two still wash up.
White phosphorus burns at 1,300 C (2,370 F) and its flames cannot be put out using water. It can cause horrific burns that often require skin grafts.
Fortunately, this time, only the jacket was damaged.