The mineral Marcasite, many times called "white iron Pyrite" or 
"fools gold", is iron sulfide. Though Marcasite has the same chemical formula as Pyrite, it is a polymorph of Pyrite and crystalizes in a different crystal system, thereby making it a totally separate mineral. Marcasite (picture to right) is often mistakenly confused with Pyrite (picture below) but Marcasite is lighter and more brittle. Specimens of Marcasite often crumble and break up due to the unstable crystal structure, and it is this crystal structure that is the main difference between Marcasite and Pyrite.
"fools gold", is iron sulfide. Though Marcasite has the same chemical formula as Pyrite, it is a polymorph of Pyrite and crystalizes in a different crystal system, thereby making it a totally separate mineral. Marcasite (picture to right) is often mistakenly confused with Pyrite (picture below) but Marcasite is lighter and more brittle. Specimens of Marcasite often crumble and break up due to the unstable crystal structure, and it is this crystal structure that is the main difference between Marcasite and Pyrite. .
Marcasite can be formed as both a primary or a secondary mineral. As a primary mineral it forms nodules, concretions and crystals in a variety of sedimentary rock, where it forms as sharp individual crystals and crystal groups. As a secondary mineral it forms by chemical alteration of a primary mineral such as Chalcopyrite. On fresh surfaces it is pale yellow to almost white and has a bright metallic luster. It tarnishes to a yellowish or brownish color and gives a black streak. It is a brittle material that cannot be scratched with a knife..
Marcasite may go through a condition known as "Pyrite decay", in which a specimen slowly disintegrates into a white powder. When a specimen goes through pyrite decay, the marcasite reacts with moisture in the air, the sulfur combining with water to produce sulfuric acid that attacks other sulfide minerals. It is unsafe to have around at this point.
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The Marcasite/Pyrite polymorph pair is probably the most famous polymorph pair next to the Diamond/Graphite pair. Adding to the confusion between Marcasite and Pyrite is the use of the word "Marcasite" as a 'Jewelry Trade Name'. The term is applied to small polished and faceted stones, usually Pyrite that are inlayed in sterling silver and sometime platenium with black rhodium highlighting the stones. But even though the jewelry trade refers to these small stones as "Marcasite", they are actually small pieces of Pyrite. In jewelry, Pyrite is used as the gemstone and is improperly termed "Marcasite" following the misnomer of the trade name. True Marcasite is never used as a gemstone in settings due to its brittle and chemically unstable structure.
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Pendants of Sterling Silver with "Marcasite (Pyrite)" stones as highlights
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For collectors pieces of Marcasite are prized in a collection. But they must be kept sealed in air tight containers so that they do not break down and become a pile of white powder. Miners, however, often viewed Pyrite and Marcasite negatively since they had a deceptive appearance, which many mistook for gold. Many based their opinions on where to dig or pan based on pieces of these minerals being in the ground or water, and too late learned that these stones were worthless. Thusly they inherited the name "fool’s gold".
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