Hi Steve, Pyrite (sometimes called fool's gold) is very simply a rock with some iron in it. It was readily available and very cheap to obtain in the Montreal region (mostly on the south shore of the island) before there were big problems that started to develop. When the backfill comes into contact with water, it oxidizes (rusts) and it expands much like the paint bubbling up on your car when it rusts. This causes pressure under the slab (or against the foundation wall) when drainage is poor. For Ambrosia, this is probably too little too late, however be wary of circles in the slab floor that look "patched" These are almost always tell tale signs of pyrite testing, and therefore be disclosed by the owner. Ian Towers Inspectrum Inc.