Try taking away the inner outlines, the lines outside every facet of the stone. If you feel you want to keep them, i suggest using different colors on them, according to how the light is interacting with that part.
I think you should be bolder with the contrast, brighter light areas and darker dark ones. Small pixel works benefit from this. Also, for practice, play with the colors' hues, give some colors a somewhat subtle blueish, purpleish, greenish, etc tint, while trying to keep the whole tone the general color you wish.
