If I may be so bold Solifuge I would suggest that the wood and stone supports could still use a bit of work (they strike me as looking a little thin and odd)... Not that I could do a better job... Just a thought if you wanted to have a crack at making an improvement or alternative...
That was actually the idea! Remember, supports are built not as permanent structures, but as temporary struts that can be collapsed remotely.
For rock type graphics I'd recommend they be based on the typical depictions of rocks in geology textbooks (yes I've taken some geology) as these are a made to both match the rocks real appearance and be recognizable. All of these refer to rough stone surfaces, smoothing and constructions are a separate issues. The following should provide enough variety to do all the layer stones once a colorizing system is in place.
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As a complete Rock Geek, this is something I've been mulling over for a while to try and find a way to do it right. I took some of your suggested textures, as well as a set of others, and added in a few others. Each category will have a generic greyscale stone texture, which we can apply colors to in order to create all the different stone types. Eventually, I'd like to have multiple textures of each category, which will be cycled through to create more natural-looking walls.
1. Flakey Sedimentary (Shale, Mica)
2. Grainy Sedimentary (Sandstone, Siltstone)
3. Clear Crystaline (Ruby, Rock Salt)
4. Opaque Crystaline (Obsidian, Bituminous Coal)
6. Smooth Igneous (Basalt, Rhyolite)
7. Coarse Igneous (Granite, Diorite)
7. Lusterous Metamorphic (Marble, Quartzite)
8. Foliated Metamorphic (Gneiss, Phyllite)
9. Nodular (Bauxite, Conglomerate)
10. Metalic (Hematite, Tetrahedrite)