Marcasite: the mineral, so-called marcasite jewellery and precautions

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Marcasite is an unstable iron sulphide, a cousin of pyrite. So-called marcasite jewellery is in fact made of pyrite. Mineralogy, safety and care.

Key points

Colour	pale bronze yellow to tin white, metallic lustre; tarnishes to brown
Mineral family	iron sulphide, FeS2 (same formula as pyrite)
Crystal system	orthorhombic (pyrite is cubic)
Hardness (Mohs)	6 to 6.5
Density	about 4.9
Streak	greyish black to greenish black
Instability	decomposes in humidity into iron sulphate and sulphuric acid (“pyrite disease”)
Known deposits	chalk cliffs of Dover (England) and Cap Blanc-Nez (France), Reocín (Spain), the coast at Kühlungsborn (Germany)
Precautions	see the safety box below: wash your hands, no elixirs, no heating

What is marcasite?

Marcasite is an iron sulphide of formula FeS2, orthorhombic. It is the “twin form” of pyrite: the same chemical formula, but a different arrangement of atoms (they are called dimorphs). It is pale bronze yellow to tin white, with a metallic lustre, a hardness of 6 to 6.5 and a density of about 4.9. It often forms aggregates known as “cockscomb” crests, or spearhead-shaped crystals.

The name has a confused history: until the 19th century, “marcasite” referred to all sorts of shiny iron sulphides. The narrower modern definition dates from 1845. Keep in mind that it is not a jewellery mineral, despite appearances.

The big misunderstanding: “marcasite” jewellery

Old or period-style jewellery known as “marcasite”, made of tiny sparkling points set in silver, is in fact made of cut pyrite. The use of the word “marcasite” in the jewellery trade predates the modern definition and has survived. True marcasite, which is too brittle and too unstable, is not used to make jewellery.

So if a piece is sold as “marcasite” by a jeweller, mineralogically speaking it is pyrite. This is not a deception: it is a trade name, simply different from the one used by mineralogists. If you are offered a “marcasite crystal” as a mineral, it is on the contrary true marcasite.

Why marcasite falls apart

In the presence of moisture and oxygen, marcasite (and some pyrites) oxidises: it turns into iron sulphate (a white deposit, melanterite) and sulphuric acid. Collectors speak of “pyrite disease”: the specimen becomes covered in white powder, swells, cracks, and the acid can eat into labels, boxes and neighbouring minerals.

To slow the process, museums keep such pieces in a dry atmosphere (below 60% humidity). A piece that smells of sulphur or is covered in powder should be isolated.

Where is it found?

Marcasite forms in sedimentary rocks (chalk, clay) and in low-temperature hydrothermal veins. The chalk cliffs of Dover in England and of Cap Blanc-Nez in France are known for their nodules. Other sites cited include the Reocín mine in Spain, where it forms cockscomb crests, and the German coast near Kühlungsborn.

Traditional symbolism

Its subdued brassy shine evokes old-fashioned elegance, contained brilliance and constancy. In the world of crystals, marcasite is often linked with pyrite (the mineral “fire”) and with the idea of slow transformation.

These associations are modern and symbolic, unrelated to any ancient tradition and without scientific evidence. The chemical instability of the stone is, however, very real: it is in fact what sets it apart.

Safety: an unstable iron sulphide

Never in the mouth, and keep it out of reach of children and animals.
No elixirs: do not let it soak in water you drink; the water may produce acid.
Humidity: the white powder that appears is acid forming. Do not touch it with bare hands, and wash your hands; store the piece in a closed, dry box, away from other minerals and from paper.
Heat and flame: do not heat or burn it; sulphides give off irritating gases (sulphur dioxide).
Dust: do not file or saw it without a mask and ventilation.
Environment: do not pour rinse water into a pond or stream.

How to use and care for it

As a collector’s piece: in a dry box, with a small silica gel sachet to absorb moisture.
Cleaning: a soft brush or dry paintbrush. No water, no soaking, no household products.
Sun and heat: avoid, especially in summer.
Knocks: the stone is brittle.
“Marcasite” jewellery: in fact pyrite, which is more stable; avoid water, sweat and perfume, which tarnish the metal and the stone (jewellery guide).

Symbolic cleansing with water or salt should be avoided; prefer sound or smoke (guide).

Recognising marcasite and avoiding confusion

Stone	System	Appearance
Marcasite	orthorhombic	pale bronze yellow, cockscomb crests, spearheads; tarnishes
Pyrite	cubic	brassy golden yellow, cubes and striated faces
Chalcopyrite	tetragonal	richer yellow, often iridescent, softer (3.5 to 4)
“Marcasite” jewellery	in fact pyrite	small faceted points in a silver setting

Marcasite without humidity or white powder can pass for pyrite for years: only an examination by a mineralogist or an analysis can establish the difference. See real or fake stone? and Our commitments.

How to choose

For a collection, prefer a stable piece, kept dry by the seller, without white powder and without a smell of sulphur. Ask about the origin and the storage conditions. To wear “marcasite” jewellery, choose pieces that have already shown their stability (well-kept antique jewellery) and store them away from humidity. For a more robust brassy shine, pyrite is a better choice.