One of the things I found most helpful is to figure out the actual geology of the rocks you're trying to model and look up reference of them. Even if you're doing something fantastical and imaginary, refer to real-life rock formations for how the small details form and layer together.
Cracks, striations, layers, crystalline structures, all are very unique to different kinds of rock and having those work correctly is not just crucial for realistic rocks, but also helps make imaginary ones convincing.
/you have no idea how gobsmacked I was when I learned that, yes, there is a significant difference between muddy sandstone and sandy mudstone.
Replies
http://www.polycount.com/forum/showthread.php?t=125857
Cracks, striations, layers, crystalline structures, all are very unique to different kinds of rock and having those work correctly is not just crucial for realistic rocks, but also helps make imaginary ones convincing.
/you have no idea how gobsmacked I was when I learned that, yes, there is a significant difference between muddy sandstone and sandy mudstone.