You really shouldn't be using rocks from outside anyways as that is extremely dangerous even once sterilized
Look for rocks from a pet store,for example in the reptile or fish section.Lava rock(true lava rock,not the smooth kind sometimes marketed for small animals and other pets)is wonderful.Smooth rocks won't do anything for the nails,while slightly rough(not sharp)rocks will actually trim nails.Even with pet store rocks you still need to at least rinse them thoroughly with hot water,and preferably freeze them for a minimum of two days,before using them.They're much safer than taking the unnecessary risk of using rocks from outdoors though,and rinsing is primarily to get rid of dust and debris from rocks getting broken up to be suitable for selling purposes
Keep in mind pumice is flat and won't distribute pressure in a good way.Flat surfaces being overused,while seemingly beneficial,are actually quite dangerous if the material is rough.This will result in sores or injury eventually,given the constant pressure applied to tiny hamster feet as would be done with sandpaper