For nether survival runs, some nether quartz tools would be really handy. Give them the same mining level as stone tools, terrible durability, Iron comparable mining speed, damage, attack speed, and enchantability, say, and I think they'd have a decent niche in both nether and classic survival. I'm not entirely sure it's in keeping with the theme of the mod, but it's not too far off, either.

If you want to get really crazy, you could allow nether quartz to substitute for feathers when making arrows? It would allow actually making feathers in the nether, and explain how pigmen make their arrows. XD

I preferred your idea of the rusty stuff as a substitute for iron or a way of getting iron, a new type of stone tool doesn't really seem like something we would add since those are already possible

True, but as described quartz tools would be significantly different from stone, in a way that I think makes them interesting. Especially with how rare blackstone is.

Also, quartz shears would be cool. much lower durability, but better mining speed than iron shears, and slightly more enchantable, too.

Quartz tools doesn't exactly seem like it fits all that well inside of Cinderscapes, but I'll certainly make a mod that does the same thing and has compatibility with Cinderscapes under @CumulusMC.