I'm wondering why tin oxide (Moh's: 6-7) and especially cerium oxide (Moh's: 6) are usually used to polish hard minerals like quartz (Moh's: 7) when a soft material (lower Moh value) cannot scratch a harder material no matter how hard you try. As I understand polishing, the polish eliminates visible scratches and replaces them with smaller scratches which are <= 1 um and are invisible to the naked eye so the surface "looks" mirror smooth. Why aren't Al2O3 or silicon carbide used when these have a hardness= 9?
Re: Why is cerium oxide used to polish quartz?
Try the links in the MadSci Library for more information on Earth Sciences.