  Optical polishing powder. 
Cerium oxide in the form of a very fine powder is used as a polishing compound for optical glass.  Supposed it works better than aluminum oxide or other abrasives in certain situations.  It's very cheap: A few dollars a pound, which tells you something about how rare the "rare" earths actually are, which is not very. 
Source: eBay seller fellows 
Contributor: Theodore Gray 
Acquired: 1 October, 2002 
Price: $5 
Size: 1" 
Purity: <30% 
Sample Group: Powders
 |  
  |  
 |  
  |