The goal of the mod is to add as little stuff as possible (I mean other than items, fluids and recipes), this is why I use vanilla stone as a source of silica for the making of silicon.
On the other hand I already added sulfur as a minable raw material, so I might add more if really it is needed for balancing and is unobtainable anywhere else.
As for the metallurgy, I'll stick to the vanilla materials (copper, iron and steel) and I will rather focus on the process to make them (pig iron crafting, blast furnaces, basic oxygen steelmaking, etc).
Not sure yet but I might also make it so the ores contain small quantities of various elements (titanium, gold or stuff like that) that you may then balance using alternate recipes a bit like in statisfactory (for example, using 2 titanium plates to make a motor instead of 4 iron plates).
Hope it's more clear now :D
