Re: Why is cerium oxide used to polish quartz?

Jim, 
In response to your question and in particular the relevance of Mohn 
hardness. I was not aware that cerium oxide had a lower value than quartz.
The Club has since its inception, as many other Clubs worldwide, used 
cerium oxide / alumina /linde 'A' and tin oxide as polishing agents.
The cerium oxide we use is optical grade but that would only enhance its 
purity and not its hardness in relation to other materials.
A major factor for not using other compounds may be the cost involved.
Silicon carbide at 800 grade, certainly from our supplier,  is not far 
short of the same price as cerium oxide which is in the region of 1000 - 
1200 grade (using the same scale). Silicon carbide is generally used in 
the lower grades 80 - 600 as grinding and lapping mediums. 
As you say polishing is the removal of visible scratches to invisible (to 
naked eye) scratches. I personally have never heard of A1203 and cannot 
comment on this compound.
The best of medium all round has to be diamond, it's clean, it's the 
hardest but by far very expensive especially in a Club environment.

I have no idea why a 'softer material' can polish a harder material so I 
have printed out your question and will post it in the Club to see if 
any of the older, wiser, members can come up with a more accurate answer.
Possibly heat generated by friction during the polishing process is a 
factor.
I hope I have been of some help with your question and if I get a clear 
answer from the Club I will post it as soon as.
Alec McCreadie