May be silly. but what happens when you apply a different material? (Perhaps save the original one first and then apply a different one). I'm assuming all will be fine then. What happens if you start a new scene and use these rocks with material, does the same thing happen?

Artwork and 3DToons items, create the perfect place for you toon and other figures!

http://www.renderosity.com/mod/bcs/index.php?vendor=23722

Due to the childish TOS changes, I'm not allowed to link to my other products outside of Rendo anymore :(

Food for thought.....
https://www.youtube.com/watch?v=pYZw0dfLmLk