I would have used PS...noise etc...this would have given a good effect. As ND also said you could use a procedural...there is noise/cloud etc within Maya. What you have is pretty good mate...nice effort...with PS you can apply motion or radial blur too so give it a rounding effect. But in the end whatever works mate...and isnt too hard to do LOL, the top rock looks good..the bottom one seems to lack "body"....
"A Darkness at Sethanon", a book I aspire to model some of the charcters and scenes