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Marcasite
Marcasite is sulfide (iron, orthorhombic), FeS₂. In the hand, it rates 6 to 6.5 on the Mohs scale, and it runs pale brass and tin white on fresh breaks. Pyrite's paler twin, silvery-brass cockscomb crests and spear-shaped twins, infamous for slowly decaying to gray dust and acid on the shelf.
The geology.
Marcasite carries the formula FeS₂. It belongs to the orthorhombic system. On paper it is sulfide (iron, orthorhombic).
It sits at 6 to 6.5 on the Mohs scale. The surface reads metallic, pale. Light behaviour is opaque. Specific gravity runs 4.85 to 4.9. The streak is greenish black. It breaks with distinct cleavage; uneven fracture; brittle.
Pyrite's paler twin, silvery-brass cockscomb crests and spear-shaped twins, infamous for slowly decaying to gray dust and acid on the shelf.
Expect to see it as cockscomb crests, spear twins, nodules in chalk or crusts.
The origins.
Low-temperature sediments: chalk nodules (England, France), Mississippi Valley (USA), Mexico.
On value, the database notes: affordable; pristine cockscombs are collector pieces while they last.
Traditional associations.
In energy work traditions, marcasite borrows pyrite's prosperity themes with a footnote of impermanence.
Recorded here because it is part of how this material is sold, not because it is a property of the material.
Spotting the real thing.
Pyrite's paler twin, silvery-brass cockscomb crests and spear-shaped twins, infamous for slowly decaying to gray dust and acid on the shelf.
Jewelry labeled marcasite is pyrite (stable, honest trade custom), true marcasite specimens need dry storage.
| Looks like | How to tell them apart |
|---|---|
| Pyrite | Same formula, different architecture, pyrite is brassier in cubes; marcasite is paler in crests and spears |
| Arsenopyrite | Arsenopyrite is whiter, heavier, in clean wedges |
| 'Marcasite' jewelry | Ironically, jewelry 'marcasite' is actually cut PYRITE, true marcasite is too unstable to wear |
At-home tests.
None of these need a lab. A 10x loupe, good daylight and a steady hand cover most of what a buyer needs to know about marcasite.
| Test | What you should see |
|---|---|
| By eye | Pale brass color and crest habit |
| By eye | Greenish-black streak |
| By eye | Watch for white-gray powder bloom (decay), isolate affected pieces |
Anything that scratches, burns or dissolves the piece is a lab job. Get it appraised rather than testing it apart.
Care & handling.
At 6 to 6.5 on the Mohs scale it handles careful wear but scratches against quartz and dust. Store it separately and take rings off before rough work.
It breaks with distinct cleavage; uneven fracture; brittle, which is worth knowing before you set it or drop it.
Warm water and a soft cloth clean almost everything. Ultrasonic tanks, steamers and household cleaners are where pieces get ruined.
Decaying specimens shed acidic dust, keep dry, isolated from paper labels and other minerals, and wash hands.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is FeS₂. Crystal system: Orthorhombic. Specific gravity: 4.85 to 4.9. Streak: Greenish black. Cleavage and fracture: Distinct cleavage; uneven fracture; brittle.
How it reaches the market
Low-temperature sediments: chalk nodules (England, France), Mississippi Valley (USA), Mexico.
What it gets confused with
Pyrite: Same formula, different architecture, pyrite is brassier in cubes; marcasite is paler in crests and spears. Arsenopyrite: Arsenopyrite is whiter, heavier, in clean wedges. 'Marcasite' jewelry: Ironically, jewelry 'marcasite' is actually cut PYRITE, true marcasite is too unstable to wear.
What it is worth
Affordable; pristine cockscombs are collector pieces while they last.
Where buyers get caught
Jewelry labeled marcasite is pyrite (stable, honest trade custom), true marcasite specimens need dry storage.
One more thing
Marcasite can self-destruct, 'pyrite disease' turns museum specimens to sulfuric crumbs, so curators store it like a slow chemical time bomb.
An education-first guide.
No product links here. We do not sell marcasite, and a guide that only covers our own shelves would not be much of a guide.
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