I'd recommend looking a few examples and going at it again. The first thing you want to look at is color; without zooming in on your rock, it looks entirely black, the colors look exactly the same. Up the contrast so the different shades can be recognized at a distance, like making the lightest shade a light grey and the next one a darker shade and so on.
Here's a good example of a rock, along with some other materials that ought to show you the kinds of things I'm talking about.
