Here are several comments from the Minerals subreddit thread:

Around the baltic sea, but also north sea, phosphor remains from bombs of the world wars are found on the beaches and are mistaken for amber and cause injuries because it starts burning. So now that you know that and you can take some precautionary measures, you could have some fun with that one.

Does it smell like almonds? Keep it in water. If it has white phosphorus in it, it will have a smoky layer on the surface and give off a faint mist like a model train engine. It will smell like astringent almonds. The stuff is the most extreme substance that I’ve ever handled by far and I’ve worked with nuclear weapons. White phosphorus makes a brine with the water like Satan’s piss. Leave it out in the sun. Ultraviolet will convert it into red phosphorus which is stable albeit slowly. The real nightmare is that it heat upon contact with air, melts into a paste like soft butter, eventually drips like candle wax and everything it has touched is about to burst into flames as hot as he sun. White phosphorus is truly the stuff of nightmares. I took on the responsibility of handling 100g of it a few years ago and I regretted every second of it for the months it took me to convert it into 60g of red P I keep as a reminder to never do that again.

Just another little thing to add here. White phosphorus is also extremely toxic. It’s a little more toxic than potassium cyanide. Where the median lethal dose for white phosphorus is between 50-100mg, vs 140mg for potassium cyanide.

After reading the comments, the original poster left a followup:

Once the hazardous waste team arrived, they took a small sample and ran some tests. Nothing showed up on their equipment to confirm what it was. They then burnt a small sample in their mobile fume cupboard and seemed satisfied that it was phosphorus.

I think they made contact with other specialist teams that deal with unexploded ordnance. They then took the decision to burn the rest. Initially they set up a small stove and gauze but the small sample they lit burnt straight through the gauze and damaged the stove. The decision was then taken just to burn the phosphorus on the ground. They touched it with a naked flame and off it went. Continued burning for nearly 10 minutes. Can see much damage it could have done if indoors (or in a jacket pocket!).

Every day I blog I learn something.