Rinse the rocks with water to remove loose dirt
Scrub them with a stiff brush
Soak them in warm soapy water
Rinse them thoroughly after washing
Use a toothbrush for cracks and small crevices
Soak stubborn dirt in vinegar and water if needed
Rinse well after using vinegar
Dry the rocks completely with a towel or air dry them
Repeat the cleaning if dirt remains