|Isotopically pure lead-207.
Separating different isotopes of the same element is incredibly difficult. In the case of uranium, this is a really, really good thing: Isolating the bomb ingredient uranium-235 from the far less radioactive uranium-238 is so difficult that only whole governments are able to do it. If it were any easier, there would be a lot more atomic bombs in the world (or maybe a whole lot less, if we had blown ourselves up already). It's hard because you can't use any chemical means of separation: The isotopes are chemically identical in every respect. You instead have to use some physical property, like the density, viscosity, etc, of the substance, which will typically very only a tiny, tiny amount from one isotope to another.
So it should come as no surprise that this isotopically pure lead-207 was created as part of the "Star Wars" research program of the US Government: In all likelihood no one else would have been able to afford the process. The idea was to create some kind of laser using a lead-vapor cell (see below). It didn't work, so now there are vials of this incredibly exotic stuff floating around eBay at prices no doubt thousands of times less than the cost of creating it.
Source: eBay seller heruur
Contributor: eBay seller heruur
Acquired: 24 December, 2003