Hi. You should probably put these requests within the same thread, rather than make new ones.
The only advice I can give is to go out with a sketchbook and draw from life. I'm a big fan of using conte or charcoal, and my pencil of choice is the ebony since it offers an incredible range in value.
Do some natural environment sketches. National parks are an ideal location, and surprisingly the local museum might have some interesting still-life setups. Begin by focusing on the structure of rocks, rather than texture. Try to make sense of the seemingly random design of various rock formations. A good artist knows how to make random and organic objects look appealing and well-designed. Debris, rocks, leaves, etc. can easily end up looking like a mess without careful observation.
I've personally found figure drawing to be invaluable when it comes to painting rocks, trees, etc. Structuring a human body gives a lot of insight into painting nature. It's important to think of things like rocks and trees as having anatomy.
Once you are familiar with drawing rocks, it doesn't really matter what tool you use to render them with. I'm not particularly a fan of realistic painting with Photoshop, but I'd probably go with the brush tool using an irregular bristle.
Here's a 2 minute quickie example. It's kind of sloppy, but when painting rocks, you want to start quick with bold contrast. Don't worry about details until later.
I put down my base color, define a bit with shadows, and then detail with highlights. The difficult thing is you are dealing with a single color througout the rock, so it's paramount that you define the edges with contrast. Hope that helps.
