Pyrite
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The mineral pyrite, or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula FeS2. Pyrite is the most abundant sulfide mineral.
Intergrowth of lustrous, cubic crystals of pyrite, with some surfaces showing characteristic striations, from Huanzala mine, Ancash, Peru. Specimen size: 7.0 × 5.0 × 2.5 cm
Pyrite under normal and polarized light
An abandoned pyrite mine near Pernek in Slovakia
Marcasite
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The mineral marcasite, sometimes called "white iron pyrite", is iron sulfide with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide
Marcasite with tarnish (8×6 cm)
Massive aggregate (c. 9 × 7 × 5.5 cm) of marcasite from the steep coast near Kühlungsborn, Mecklenburg-West Pomerania, Germany
Marcasite crystals forming a cockscomb-shaped aggregate. Reocín Mine, Reocín, Cantabria (Spain).
Iridescent cluster of marcasite crystals (3.3 × 2.1 × 1.4 cm)