I does nothing to my water chemistry it was formed naturally around here, mine near here, by mount lassen orginally lava raining from the sky, cinder comes in golfball sizes to sand to dust. basically it is stable unless you have a powerful kiln.
It is black , the red is what has iron in it the black Ive had with my other fish and snails and frogs and it is fine.
I like it because it is cheaper than dirt for me.
Thanks hebily for the reassurance.