Cells are incredibly complicated structures, containing organelles that can only be viewed in any detail with electron microscopes.
A rock on the other hand is simply a mixture of various mineral crystals. They can be large crystals, easily seen by the naked eye, such as in granite; or they can be very small, as in basalt.
A rock is very very simple in composition in comparison to a cell. A bit like comparing a sand dune to a forest (and all the organisms living in it).
