I think that if someone else had written a story with rocks being important to the plot, it would be unlikely to be a breach of copyright unless you started using similar characters and places. It all depends, of course. (I am not a lawyer, btw.)Co-incidences of plot have happened before. Even JK Rowling had several little known authors claim to have 'invented' Harry Potter and the Muggles first.
I wouldn't be too concerned, Ar-edain37. Your own work will naturally be different from anyone elses, because every author has his own individual 'voice'. Plagiarism has to be deliberate and is difficult to prove in any case.
"May the Angels Guide"