|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