  Nut and bolt, broken. 
Purchased from Neil Lipson through eBay.  This bolt was said to have been from a high-temperature pressure vessel of some sort.  If you look at the broken surface in the high-resolution picture, you can clearly see that it's not like any steel, at least not like any I've ever seen.  The melting point of molybdenum is a good 1000C higher than steel, and it's pretty strong stuff. 
 
Analysis by x-ray fluorescence spectroscopy at the Center for Microanalysis of Materials, University of Illinois (partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439) indicates that this bolt is approximately 92% molybdenum with the remainder a mixture of cobalt and chromium. 
 
So, in case you had any lingering doubts about whether someone would pay $13 for a broken 1/4-20 bolt on eBay, the answer is yes. 
Source: Neil Lipson 
Contributor: Theodore Gray 
Acquired: 15 August, 2002 
Price: $13 
Size: 1.5" 
Purity: 92%
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