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Physical Characteristics
Iron sulfide (pyrite) is a brassy yellow mineral with a metallic luster, occurring in opaque cubic, octahedral and dodecahedral crystals or combinations of these forms. It's commonly found in nodules. Sometimes the nodules have been flattened into shiny circular "dollars" by geologic forces. Iron sulfide may resemble gold in appearance, but it is hard and brittle, not soft and malleable like gold. It has a specific gravity of 5.1, heavier than average for metallic minerals but much lighter than gold. It rates 6 to 6.5 on the Mohs hardness scale, about as hard as a good steel file, and much harder than gold. On the mineral streak test, it leaves a greenish-black streak unlike the yellow streak left by gold.
Pyrite Uses
Because it is pretty and lustrous, iron sulfide is used in jewelry where it's known as marcasite. It's also a favorite with mineral collectors because of its luster and its interesting crystal shapes. Pyrite was mined in the 1940s as a source of industrial sulfur, but that activity stopped after industry learned how to extract sulfur from the hydrogen sulfide gas recovered from natural gas wells. Pyrite is used to make sulfur dioxide for the paper industry and commercial sulfuric acid for the chemical industry. Pyrite consists of 46.6 percent iron, higher than commercially-mined iron oxide ores, such as hematite, but it isn't mined as an iron source because it doesn't occur in massive, easily-mined concentrations and the sulfur content makes pyrite very difficult and expensive to refine into pure iron.
Pyrite as Semiconductor
Iron pyrite is an electronic semiconductor and in the early 20th century was used as a radio-wave detector in crystal radio sets. In the 1990s it was studied as a material for photovoltaic solar cells. While less efficient than silicon in converting sunlight to electricity, pyrite is abundant worldwide and much cheaper to extract than silicon. Cost is important when considering solar electricity production in trillions of kilowatt hours. But pyrite is difficult to work with, and its low efficiency up to now has offset its low cost.
Metaphysical Pyrite
Iron pyrite's elemental content, crystalline structure and resemblance to gold have led some people to believe it has metaphysical properties as a healing stone. Its resemblance to gold is supposed to attract wealth. Its crystalline structure is supposed to improve mental focus and clear thinking, reduce anxiety and frustration, and generally stimulate mental processes. The elemental iron and sulfur supposedly facilitate transfer of oxygen from lungs to the bloodstream, and confer general benefits to the respiratory and circulatory systems. The purported benefits come from wearing or handling the mineral, or by keeping it close to you like on a desk or nightstand.