Pyrite or fools gold contains iron sulphide which means that it is not good for an aquarium because,
"1. It has a strong acidifying effect, i.e. it drops the pH.
2. In addition to iron it sometimes contains small quantities of other more dangerous heavy metals such as lead, zinc, and copper.
3. Because it acidifies the water it can 'unlock' heavy metals into their toxic free-ionic forms."
IMO I wouldn't attempt using it in the aquarium without knowing fully what type of rock it is.