Sand and grit proof mac case

Probably the closest to what you are asking for here is a skin or wrap, and a bag or sleeve, and not a case.

DBrand offers skins and wraps for most (all?) recent MacBook Pro models, among other vendors offering similar.

Sand in many areas tends to have quartz, which scratches most other materials.

They will all do this since there must be some gaps to allow the use of the laptop. I tell people to avoid any cases for this reason, plus most of the cases I have seen develop cracks which makes the case loose where many times I've almost had the laptop slip out and drop to the floor. Just get a nice laptop bag to carry the laptop around.