The 67-year-old was walking along the Baltic coastline on Monday afternoon when he spotted a pale yellow stone he recognized as being amber – which often washes up on north-eastern German beaches.
But so do chunks of white phosphorous since they were used in World War II bombs. And to the untrained eye, the two can look similar.
Unfortunately for the man, his find was white phosphorus, not amber, police said in a statement.
After slipping it into his pocket and continuing on his walk, it burst into flames and set him on fire.
Despite being in the relatively remote area of Steine, near Kiel in Schleswig Holstein, there was an angler nearby who spotted the burning man and called an ambulance.
White phosphorus cannot be extinguished with water, and the man suffered third-degree burns. On Wednesday he was still being treated at a specialist clinic.
Schleswig Holstein police confirmed in a statement that the Hubertsberg beach had been shut off and was being searched for further munitions.
The state's office for criminal investigation is warning walkers to never pick up something if they are not sure what it is.