If you want to make them to look 16-bit, then you're going to have work with limited amount of colors. You should first study other types of rocks and draw them out in a 2d platforming perspective, Sonic style. Well, I hope there's a style for that.
Also, if you're going to make graphics on your own, then you should look up some pixel art tutorials and learn the basics.
like in this link: http://www.natomic.com/hosted/marks/mpat/
Hope you understand and take my advice.
EDIT:Can you at least post your rocks here. I want to see what you're working on.