Are these rocks for mere set decoration only?
Or are they going to be climbed on, sat on, and used
by the actors, etc?

For mere decoration: chicken wire covered with
several layers of papier mache does the trick.
Instead of strips of newspaper, strips of muslin
create more texture and will last longer. (However, it
is still rather flimsy and will suffer from being banged
about.) Paint with different shades of grey mottled
with other colors.

For climbable rocks: a wooden framework would
need to be constructed (freeform shapes of 3/4
plywood nailed together with 2x4's), then draped with
wire mesh, covered in sturdy fabric such as burlap,
and sprayed with a fibreglas material. (someone in
the auto-body repair business might be helpful with
this process) While not exactly "lightweight" these
rocks are more easily moveable than actual
boulders of the same size. Casters can be
concealed within the framework, if so desired.

The same technique can be used to make a sittable
or climbable tree-stump as well, if you take the time
and effort to staple the burlap into irregular, vertical
pleats.

This is just for sitting there rocks (that no one touches)

If painted properly you can use foam for rocks - just make sure no one bumps into them or you can sure tell they're fake :)