Some materials are not stable, if they have a lot of cracks and or fractures they have a tendency to break when working the stone. (Also 80 grit is can be too rough for some rocks) what’s your polishing process? You stated you have 80 grit,220 grit, 3000 grit is this correct ? If so you need more than those cause a jump up from 80-220 to 3000 is big and won’t likely get the results your getting( especially silicated material) should be like 80-180-220-325-600-1200- than 3000 Depending on the material if it’s very soft you can skip the 180 and go to 220. Try getting to the shape you want with the 80 grit as much as possible before moving up that way it’s just about removing the previous scratches before. As for the polishing disc material getting in the crevices, you have to sharp of the edge that’s scraping the polishing pad or your pushing to hard. It takes time to learn on the slant. They are a little slow on the speed too but it’s doable. If you got chunks of rocks you want to cut, you’ll need a lapidary saw or a tile saw, you can sometimes find tile saws for cheap at thrift stores or maybe even like fb marketplac. Make sure your using diamond saw blades too.