To expand on what eishiya said, try to think about the material and it's environment more too!
Stone can be filed down pretty well, but it's a rough, brittle material! And in space, it'd be prone to things like chipping from small asteroids to cracking from general wear!
Also, try to think your color choices through more! I lowered the contrast and coloration on some of your greys to produce this stone cube, so you can see the things I did with it better!
Notably, I implemented those craters and pockmarks as well as some "bumps" implied by the lighter pixels and the shadows they cast from the light source.
Start with a smooth two-tone cell shaded structure, and try to work that texture in -sparingly-!
Pixel art is a very restrictive medium, if you try to push too much texture into a given space you'll wind up obfuscating what it's supposed to be. Less is very much more!*
(*This is not always the case, but it's what I try to work from most when I'm doing pixel art. Reducing colors, textures, and details does a lot for making it clear what you're looking at when done properly!)
