This mineral, Thorite, appears in my Photographic Periodic Table Poster representing francium, because this highly unstable element can't reasonably be photographed. The rock probably contains on the order of a few atoms of francium at any one time, as part of the complex decay chains of the thorium and uranium that make up a much larger fraction of the sample. In no meaningful way is francium itself visible in the sample, but sadly that's about the best you can do with an element like this. The sample photograph includes text exactly as it appears in the poster, which you are encouraged to buy a copy of.
Source: eBay seller mineralman999
Contributor: Theodore Gray
Acquired: 3 June, 2005
Text Updated: 4 May, 2007