While mountains may be somewhat alien to me, I find my palette knife technique serves me well in creating an interesting rocky texture.
It's all about 'broken color'. Used effectively, a knife allows you to 'float' bits of pure color over shades of gray. So, suddenly a gray rock can have blues! and pinks, and golds! And it really, really is there. It just takes some reflection to see it, tease it out, and some expertise to paint it.