All rocks are hard to do. There is no form with rocks and they demand a very random approach. Trees and skies fall into the same category – there is no set way to do it.
The tricky thing with rocks is the lack of proper lines. When you try to form lines like a building, the rocks can look artificial and not natural.
I believe it’s better to approach them from a random perspective. Once you have a basic outline, you can allow the pencils to work freely as you can. Ideally, you’re looking at impressionism if you can. The more impressionistic kind of work you do, the better you become and the more you understand how to not create form.
Also, it’s not just the random approach to the drawing but also the colours. The colours have to be treated in the same random way. Create a rough drawing and use the pencils to create it’s own form on that rough drawing. As always the best way to explain this is by showing you and on the member’s site there is a video showing how to tackle this tricky subject.