the rock you see is often crystallized from magma. . . so they are sometimes the same thing. The source of the rock affects the forms that it takes when it is shattered or eroded to show in cliffs/boulders/etc.
arrangemonk, igneous rock is not really crystalized like that, and sedimentary rock looks nothing like it - it is deposited in layers. Check out some more ref for getting believable rock forms, you will see repeating shapes of varying size.
Woah JI this transluency shader looks impressive! I'm usually not a huge fan of the "balloon filled with water" kindof look that overdone transluency often gives, but your take on it looks great. Can't wait to see that applied to thin wing membranes, crystals, or even just to nicely present a sculpted model like on your…