Sintered button. 
This is a compressed and sintered button of rhenium powder.  It could be melted down into a solid button in an arc or e-beam furnace (see above), but it's interesting to keep a few of these around in pre-melted condition for comparison.  I got this one from Oliver Sacks on a visit to New York, during which we visited a company in New Jersey to melt his kilogram of iridium down into a solid slab. 
Source: Oliver Sacks 
Contributor: Oliver Sacks 
Acquired: 4 August, 2004 
Price: Donated 
Size: 0.75" 
Purity: 99.9%
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