If you already loaded the rocks as ecosystem-
In the world browser click on the materials tab( 2nd from the left) and you'll get a list of all materials including eco materials.  Double click on the material you want and you'll get into the material editor and replace or change the material.
Alternatively, just load one rock, edit its material, save as vob and then load this rock(as object) in your eco system.

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
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