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"....
cheers bullet
bullet1968
"A Darkness at Sethanon", a book I aspire to model some of the charcters and scenes