Blackthorn, same tree/hedge where you pick your sloe berries for gin! The berries have a blue haze covering them, some kind of yeast/fungus, the same stuff is all over the prickly growth and when you get pricked the tip usually breaks off. Therefore, when you get spiked that stuff goes under the skin and stays there with a small piece of the spike causing infection and pain…If you’re really unlucky you can develop septasimia!
http://druidry.org/obod/trees/blackthorn.html