This question intrigued me so I looked around the net. Almost all answers came back to say don't do it.
Here's one of the best explanations I found:
"Fool's gold, also known as pyrite, has a strong acidifying effect, so it drops the pH. In addition to iron, it may also contain heavier metals such as copper, zinc and lead, and because it acidifies the water, it can "unlock" heavy metals into their toxic free-ionic forms. Oxygen and natural bacteria in the water oxidize the sulphide component of pyrite to sulphate/sulphuric acid, and the low pH water produced then activates the iron and any other heavy metals present. It is NOT safe for aquarium use!"