  Silicon mirror. 
This is a mirror designed to reflect something other than visible light: Perhaps someone will enlighten me.  It's obviously very flat and shiny, but not too good as a visible light mirror because silicon absorbs a lot of the visible light striking it. 
Source: eBay seller flabster 
Contributor: Theodore Gray 
Acquired: 20 June, 2004 
Price: $10 
Size: 1.5" 
Purity: 99.9999%
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