[quote=@Great Sogeking]
It's just a body-hardening Quirk that applies one arm per transformation, yeah. As in, like, it's not a mix of flint and steel throughout one arm. It's just one arm being steel and one arm being flint since its primary usage is supposed to be causing sparks that can ignite things.
[/quote]

Inertia's character sort of already does that, although rather than open flame the focus is on explosions. While the body-hardening would put a different spin on it, I'm afraid I'd really prefer if there were as little overlap as possible between the powers, sorry. I do like the idea of a body-hardening Quirk that uses different materials, though--maybe use some metallurgic knowledge of alloys or something to figure out another way to apply this kind of power? Is there such a thing as two metals that become magnetized or electrified when rubbed against each other, or something like that?