personally i dont much care for it
i made this along time ago for someone else, but it might help you too
id suggest first figuring out exactly the forms of your rock - picking out which lines are edges, and which lines are corners. corner lines are darker, while edges are highlights
throw on your base colours
establish your shadows and highlights
colour your rock to look either plastic or metally
use dithering as a technique to add texture and roughness, opposed to a technique of blending colours
then play around with it abit
your rock is too soft..
use more block colours, instead of dithering over 1 base colour to create highlights and shadow
each shade should become a block colour and then use dithering only to blend slightly and to add texture
thats some quick crit from me
hopefully that image may be useful