Pyrite is not a rock, but a mineral. It's a member of the sulfide family and is made up of iron and sulfur. Its chemical formula is FeS2. Pyrite's name comes from Greek and alludes to the fact that sparks form when it strikes iron.
Pyrite goes into forming igneous, metamorphic and sedimentary rock. It is often found in crystals of various shapes and is often dark yellow and lustrous. This leads to its other nickname of "fool's gold," since many prospectors mistook it for true gold. Interestingly, it is sometimes found with gold.
Pyrite is very heavy and hard, with a rating of 6 on the Mohs hardness scale. It doesn't dissolve in hydrochloric acid but does dissolve in nitric acid if it's in powder form. When pyrite burns, it gives off fumes that smell of sulfur. It's also unstable in nature and eventually turns to sulfide and rust when exposed to air. Pyrite is even attacked by bacteria, which hastens the process. The sulfide that results from the decomposition of pyrite can dissolve in water to create sulfuric acid. When it's used in construction, the breaking down of pyrite can make the building unstable.
However, pyrite is used in lithium batteries and is an ingredient in the making of marcasite jewelry.