I think I know what tutorial you've been looking at, and my critiques apply as much to that tutorial as they do to your rocks:
You have highlights around every edge of each segment. This makes it look flat. Have highlights only on those edges that are closest to the light source. All the other edges are facing away, and should not get highlights. If the segments are meant to be round (like river pebbles), then there would not be any edges, the highlight would instead be a small tick of light running in the longest direction of the pebble.
Have you ever seen a clump of rocks like that? Rocks don't aren't sticky so they don't clump together (unless they're held together by some sort of mortar, which you've not shown here). And if these are meant to be individual rocks rather than clumps of them - again, what is it? There is a whole class of rocks that are made out of other rocks (conglomerates), but they don't look like that - they have a "mortar" in which the bigger rocks are stuck, and they don't all align to form a smooth surface. I realise this is texture practice and not an attempt to draw a realistic rock, but if you draw these kinds of rocks in any other context, these same problems would come up - these just don't look like rocks, because they don't have the texture of the kinds of rocks most people are familiar with. And I can't blame you for that, because it's that tutorial that's teaching you not-so-good things (like highlighting everything everywhere xP).
Haha from what I've seen, some well established pixel artists here don't like that set of tutorials, since it teaches some bad habits. This one is really solid and teaches you a lot about the language of pixel art:. http://pixeljoint.com/forum/forum_posts.asp?TID=11299&PID=139318#139318 Try not to feel overwhelmed, and keep practicing! I'm fairly new myself so (: