If we're talking about Peak District Gritstone, I'm sure it'll be fine. It's a very durable building material. The inert silica grains are chiefly bound together with iron oxides which gives it its colour. I wouldn't worry too much about gritstones bound by calcium either, unless they're very soft. Seiryu and ryuoh stone are really just limestone (calcium carbonate) in various states of metamorphosis, and both are very popular aquascaping material. Sure some affect water chemistry but not enough to worry about, that is unless you're keeping specialist blackwater species like chocolate gourami.