A very interesting specimen of pyrite that has pseudomorphed pyrite. 

 I am fairly sure that the numerous small crystals are not epitaxial growths, as fractured surfaces on other pieces show internal voids between the replacement crystals.  Either way, these are very unusual specimens-- I obtained two on my first trip to Peru a few years ago, there was another find a few months ago. 

The many reflections and varying topography makes this piece hard to focus and single out the main pseudomorphed crystals.

It is a lot more distinct and less jumbled in person... some of these are not too photogenic.

I had a several on my shelf in Tucson and sold all but one... on the internet however, they tend to sit.  This probably has something to do with the fact that they come across in pictures as massive jumbles of tiny crystals.