Pyrite is iron disulphide, FeS₂, the most common sulphide mineral in the Earth's crust and the source of the colloquial name fool's gold for its brassy yellow colour and metallic lustre. The mineral occurs in nearly every geological setting, from sedimentary shale to igneous and metamorphic rock to hydrothermal veins, and it appears in the gemmological literature in three principal contexts: as an inclusion in other gemstones, as a fashioned material in its own right (historically as marcasite jewellery), and as the source of the staining and decomposition that complicates conservation of certain mineral specimens. The name comes from the Greek pyrites, meaning of fire, in reference to the sparks struck from the mineral when used with flint as a fire-starter.

Composition and crystal habit

Pyrite is cubic, with crystals commonly forming as well-developed cubes, octahedra, and pyritohedra (twelve-sided forms with pentagonal faces unique to the iron-pyrites group). The cubic crystals often display striated faces, with striations on adjacent faces oriented at right angles — a diagnostic feature of pyrite that distinguishes it from chalcopyrite and gold, both of which can superficially resemble it. Massive, granular, and botryoidal habits also occur, and pyrite frequently replaces fossil organic remains, producing pyritised ammonites, wood, and other fossils prized by collectors.

Hardness is 6 to 6.5 on the Mohs scale — significantly harder than gold (2.5 to 3) or chalcopyrite (3.5 to 4) — but pyrite is brittle and fractures conchoidally. Specific gravity is around 5.0, lower than that of gold (around 19.3) but higher than most rock-forming minerals. The streak is greenish-black or brownish-black, in contrast to gold's yellow streak, and a streak test on unglazed porcelain remains the simplest field test for distinguishing the two. Pyrite is paramagnetic and slightly conductive of electricity.

Pyrite as an inclusion

Pyrite occurs as an inclusion in several gemstones, with its presence ranging from desirable to merely characteristic to actively damaging depending on the host. In lapis lazuli, golden pyrite flecks are considered an essential feature of the finest material — the classical visual contrast of deep blue ground with scattered golden cubes is a defining quality of fine Afghan lapis from the Sar-e-Sang mines. In Colombian emeralds from Muzo and Chivor, small cubic pyrite crystals are diagnostic of origin and are recognised in the standard inclusion photoatlases as positive indicators of Colombian source.

In other contexts, pyrite inclusions can be undesirable. Where pyrite intersects the surface of a host gem, it is vulnerable to oxidation and may produce rust staining or even bulging and fracturing of the host as the iron sulphide expands during decomposition. This is a particular concern in fossils and certain mineral specimens stored in humid conditions, where pyrite oxidation produces iron sulphates and sulphuric acid that can destroy the host material entirely. Conservation of pyrite-bearing material requires controlled humidity, typically below 30 percent relative humidity for vulnerable specimens.

Pyrite as a fashioned material

Pyrite has been used in jewellery since antiquity, most famously in the so-called marcasite jewellery of the Victorian and Art Deco periods. Despite the trade name, marcasite jewellery is almost always faceted pyrite rather than true marcasite — the latter is a different polymorph of FeS₂, orthorhombic in symmetry, considerably less stable, and prone to crumbling on exposure to humidity. Faceted pyrite, set in silver and used as a diamond substitute in evening jewellery and mourning pieces, became fashionable in eighteenth-century France and remained popular through the nineteenth and early twentieth centuries.

Pyrite cuts well on standard lapidary equipment but suffers from brittleness and a tendency to chip at the girdle and culet during setting. The metallic lustre and high reflectivity make rose cuts and small brilliants the most common faceting styles for marcasite jewellery, with the stones typically set in pavé or cluster arrangements that mask the relatively low individual brilliance. Modern marcasite jewellery continues the tradition, often with calibrated machine-cut pyrites set in silver or rhodium-plated silver mountings.

Pyrite-bearing varieties and decorative material

Beyond the marcasite-jewellery tradition and inclusion contexts, pyrite is the principal mineral in several decorative materials traded in the lapidary market. Pyrite-rich shale slabs are sometimes polished as ornamental stone. Pyritised ammonites from England, Germany, and the Volga region of Russia are widely traded as fossil specimens and are sometimes set as jewellery. Boji stones (a trade name applied to pyrite-bearing concretions) have been marketed as new-age curiosities since the late twentieth century.

Iridescent pyrite, sometimes called rainbow pyrite, is a relatively recent introduction from Russian deposits and shows surface iridescence due to a thin oxidation layer. The phenomenon is essentially identical to thin-film interference on heat-treated steel and other metals, and the iridescent surfaces are typically a few hundred nanometres thick.

Stability and care

Pyrite's principal vulnerability in the gem and jewellery context is oxidation. In humid environments, pyrite reacts with atmospheric oxygen and water vapour to form iron sulphate and sulphuric acid, a process accelerated by the presence of certain bacteria and by the surface area exposed in finely divided or cleaved material. The decomposition is sometimes called pyrite decay or pyrite disease and is a serious conservation problem in fossil and mineral collections. Marcasite jewellery in long storage can show oxidation discolouration of both the pyrite stones and the surrounding silver mountings.

Storage in dry conditions, ideally in sealed containers with desiccant, slows the process. Once visible decay has begun, conservation interventions include surface cleaning with ethanol, application of consolidants such as Paraloid B-72, and storage at very low humidity. Affected jewellery should be evaluated by a conservator before any aggressive cleaning is attempted.

Identification

Pyrite is identified by its brassy yellow colour, metallic lustre, cubic or pyritohedral crystal habit with characteristic striated faces, hardness of 6 to 6.5, specific gravity around 5.0, greenish-black to brownish-black streak, and lack of malleability. The combination distinguishes it from gold (much higher specific gravity, much lower hardness, malleable, yellow streak), chalcopyrite (lower hardness, deeper brassy colour with iridescent tarnish), and marcasite proper (similar properties but orthorhombic crystal habit and less stable). Pyrite is non-magnetic in the household-magnet sense but is weakly paramagnetic; some pyrrhotite-bearing pyrite specimens are noticeably magnetic owing to the pyrrhotite content.

Further reading

• GIA Gem Encyclopedia — pyrite and iron sulphides
• International Gem Society — pyrite
• GIA Gems & Gemology — inclusion mineralogy and lapis lazuli articles