There some way fo full fill an huge area with a certain block or more only underground as an underground biome?

I'm making a geologist realistic mod and I've been with many troubles trying to figureout how to distribute rocks in natural form. I was trying to creat a custom biome, but it must be only underground, because overground must be random and compatible with bop. I know this is kind possible because on Better Geo it's happen, but without creating a nem biome, any ideias of how can I do it?