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.
Igneous Extrusive - Fine grained suggest you use a speckled appearance, these will all end up in shades of gray from light colored rhyolite to near black basalt.
Igneous Intrusive - Courser grained like granite but still speckled, just use larger specks.
Glassy - used for obsidian, use light and dark lines like that on a glass block but with some disorder to suggest roughness.
Carbonate - carbonate rocks (flux) are always depicted with parallel horizontal lines and semi randomly spaced vertical lines giving the appearance of a brick wall built of blocks of varying width.
Sand Stone - stippled dots would be good hear but use fewer dots and more background then Igneous Extrusive.
Slate - long thin unbroken parallel lines typically depict slate
Shale - slightly thicker slightly undulating lines that vary in thickness
Foliated - many metamorphic rocks look like this, theirs a strong waviness and thick light/dark bands
Conglomerate - heterogeneous blobs of various sizes with a little background color showing through
Fine grained sediment - used for silt and mud stone these are almost completely flat, just a few speckles and color shades in horizontal bands give them some character.
Crystalline - for rock salt and many of the cluster forming minor minerals, has a crackly appearance from angular crystals.