Rock would look better than broken concrete. If you put down a geotextile fabric before placing the rock, it will help prevent the rock from sinking into the banks, and also help keep weeds from growing up between the rocks. If the rocks are partially submerged, they will provide habitat for invertebrates (i.e crayfish).

I don't know if a rock slinger truck is in your area, but if so, that'd be a quick way to do it if they can get to your pond.
http://www.youtube.com/watch?v=iYEavO9cudk

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).