A cabinet specimen consisting of iridescent marcasite aggregates from the 2009 find at the Conco Mine, located in the outer suburbs of Chicago.  

While the bulk of the material available from the mine consisted of marcasite included calcite collected later on, a single vein of this iridescent marcasite was discovered by a miner who had the foresight to save specimens prior to blasting.

More iridescent than the pictures show!