Pyrite, also known as "fool's gold" is the name given to a naturally occurring iron sulphide mineral. It is a brassy yellow in colour, sometimes tarnishing to give a rainbow effect. Like cuprite (in a previous Mineral Spot), pyrite is a member of the cubic or isometric crystal system and normally occurs as cubes or pyritohedrons (where each crystal face has five sides - very unusual in crystals).
Also like cuprite, occasionally, it grows more rapidly in one direction and forms long capillary crystals.
This specimen is from Flinders in Victoria and has 'impaled' blobs of a clay mineral.