Pyrite

Pyrite is an iron sulfide and one of the most common sulfides. 

Pyrite has the nickname of "fool's gold" from it's brassy golden color and very small pyrite can contain gold however if you can see the mineral structure there is most likely no gold. Gold forms in layers in microfine pyrite. 

It occurs in many different geologic settings including hydrothermal springs, sedimentary and metamorphic stones. Sometimes iron pyrite forms concretions and replaces fossils. A truly versatile stone.

Pyrite can be unstable and oxidize and decompose in humidity and airflow.