Going off of what Yalash said, remember the lines/edges. Each edge will create a different shading. If you were making spheres then the gradual shading would be fine, but gems have a cut and will have different shades on each face.
Because I don’t have the time to do all my own graphics ;) I’m already working on another game which is going to be released (Probably won’t be played by many people but whatever, shits and giggles). If I had the time and the ability to do good pixel art I’d invent my own Pokemon style game with all original content and original ideas.