With that most recent one, I would advise playing with the depth of the stones a bit more. They appear a bit flat and the contrast could be reworked a bit. I think you have a great start here and it's been a fun time editing the image to help out.

So if we take a look at this, we can barely distinguish your two darkest colors. This may just be a difference in monitors but it's important in pixel art to think about the visibility of your colors. Just raising the brightness of your second darkest color helps out a lot.

So the next thing I noticed is that you don't seem to like your food to touch each other on the plate. Sometimes rocks are just pressed up against one another and are just gonna have to get along. Also, the extremely varied rock sizes is part of what contributes to your grid issues, but the best way to fix the grids is using several varied tiles (which I know is something that I always forget or just don't want to do) with the texture you are trying to create.

Lastly, as you can see in the edit, I wanted to work with the lighting and color a little bit. Experiment more with this, as it can really make your pixels pop out more or recede into the background, depending on the desired effect. The midtones are emphasized more and the darks aren't as prominent; they merely serve the effect of shadowing rather than outlining the rocks. The lone tile on the left and the middle column are the greyscale from the original, and the other two are my own experimenting with color a bit. Almost no two are exactly the same hue, though they are similar in saturation. I'm not the best pixel artist in the world, but I hope that I helped in some way.
And yes, I'd say it's usable for a game. Just be sure that with the large contrast in your lightest and darkest colors that this does not conflict with any foreground images.