Wow can't believe it
; I nearly brought a box of that stuff too!
I suspect the metal is Zinc; which is sometimes a by product of carbon when it is chemically activated. I wonder if your batch had an unusually high amount of residue?
After reading this; i ran a magnet over my Marineland (Black Diamond) carbon and sure enough their are traces of metal. Worth considering if this is why i have been battling tip burn?
I think this could raise a few eyebrows!
Simon.