Also, if you're baking a normal map from those high-resolution sculpts, then it would make sense for you to sculpt the striations into those. Another reason to consider using tiled textures, instead of sculpting unique whole-rock normal maps. What's the current texture resolution you're using for each rock?
I like the variations, but the mid-level shapes are looking all a bit blobby, compared to your refs. I think you could be better served by using lower-poly meshes, with tiled textures and multi-texture blending. I'm reminded of Anthony Vaccaro's work on Uncharted 3, for example. https://www.artstation.com/artwork/rZAvJ See…
Good start. But you need to get those horizontal striations in there. Right now your rock textures just read as semi-random noise, rather than the geological structure of sedimentary layering as seen in your references. It's actually a really cool process, how these types of rocks formed over…