Rocks used to be one of my least favorite things to paint. My knowledge of rocks exceeded my painting skill by a fair margin. A downside of having an M.S. degree in geology. I knew when rocks didn't look "right", but had a hard time "fixing" them.
Once I decided to approach them as I would any other subject matter, painting rocks became easier for me. Still, almost every time I paint them, I soon get to the muddy section when they look like nothing. That is when I need to step back, formulate a strategy and attack each section slowly, working on it until it looks rocky. I hit that muddy stage in this painting once I had blocked in all the color. The metamorphic nature of the rock just wasn't coming through. Step back, deep breath. Let's start on the lower right section and build up from there.
Below you can see the rock basically done although there will be some tweaking as I continue the painting.