My teenage son says black, not charcoal, blackThat doesnt sound good. I will hyper-torch some this evening and see what happens...green bad
check this out. Bunsenite, the mineralogical form of Ni(II)O....
https://en.wikipedia.org/wiki/Nickel(II)_oxide#/media/File:Bunsenite-bem-22a.jpg
edit: of course Ni salts are coloured, usually green also, but this is just heated in air so it is likley to be the oxide I think (could be the carbonate maybe??)
edit2: not the carbonate as that is converted to oxide and CO2 with heat...
Oh, it flakes some