  Thick disk of solid beryllium. 
This is a slice of pure beryllium, probably x-ray grade meaning pure enough to allow x-rays through.  Not cheap at all, no sir, not cheap. 
Source: eBay seller chifizica 
Contributor: Theodore Gray 
Acquired: 15 April, 2009 
Text Updated: 15 April, 2009 
Price: $430 
Size: 6" 
Purity: >99%
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