Autunite, if it is not kept in a humid environment, tends to degrade due to loss of water from the crystal matrix (see formula below, which indicates there are 10 molecules of water associated with each atom of uranium). This sample flaked apart as I was trying to mount it for photography, dropping little leaves of radioactivity everywhere. Still, quite pretty.
Source: eBay seller dr**zarkoff
Contributor: Theodore Gray
Acquired: 10 June, 2005