Mostly, your shading is very good. I can see you chose to pick a light source and shaded details, intead of pillow shading like most beginners do. I did it too(with opacity lol). With shading, what you really need to do is define the muscles better. You will see in the image that I will provide. Your palette, however, is not so great. It isn't horrible, exspecially the blue palette, but still pretty bad. All you modified is probably Value and Saturation. You need to edit the Hue. This is called Hue-shifting, and you can learn more about it here. Your skin palette needs hue-shifting also, but I am not the best example of this. I thought I could get some good practice by modifying (spelling error, most likely) yours. And help you with your pixel art. Other thank that your piece is very good. You also seem to know anti-aliasing. So, overall your sprite is very good for a beginner, but you still have your palettes to improve. This is my opinion, and I don't think I am the best fit to rate your art, so you should allow someone else to help you also. Well Cure's take is better than mine, so listen to his...
                                         

I'm working on my take on your sprite, just a moment, I'll get to it.

Yes, I've been trying to make my own palettes that I can reuse, but they are failing.. beserkx33 you shouldd use hueshifting maybe thats your problem

Honestly, I liked the first one because the dithering added a sense of the crystal being CLEAR. It was really cool.