Pyrite also known as iron pyrite and sometimes fool's gold is a popular mineral that crystallises in different forms. Some of the most common crysal habits are cubes, clusters and octahedrons but it also crystallises in massive form. In this formation crystals grow together to form one large mass so have no distinctive external shape.
Another dark mineral also seems to be present in this piece but it's difficult to tell for certain what it is. It is however likely to be a sulphide mineral.
Pyrite occurs worldwide in small quantities within sedimentary, igneous and metamorphic rocks. The Rio Tinto mines of Spain produce vast quantities but Bolivia, Brazil, Peru, Japan, Canada, Italy, Norway, Portugal, Greece and Slovakia have also been known to produce fine specimens. This piece comes from South Africa.
Although a relatively hard mineral pyrite is brittle so will chip or shatter easily.