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Siderite
Siderite is carbonates, FeCO3. In the hand, it rates 3.5 to 4.5 on the Mohs scale, and it runs tan-brown and coffee brown. Brown rhombohedra with gently curved, saddle-like faces and pearly sheen, calcite's heavyset iron cousin, half again denser and slow to fizz.
The geology.
Siderite carries the formula FeCO3. It belongs to the trigonal system. On paper it is carbonates.
It sits at 3.5 to 4.5 on the Mohs scale. The surface reads vitreous to pearly. Light behaviour is translucent to nearly opaque. Specific gravity runs 3.96. The streak is white to pale yellow-brown. It breaks with perfect rhombohedral cleavage; uneven fracture.
Brown rhombohedra with gently curved, saddle-like faces and pearly sheen, calcite's heavyset iron cousin, half again denser and slow to fizz.
Expect to see it as curved rhombohedral crystals, saddle-shaped clusters, botryoidal crusts or massive beds.
The origins.
Sedimentary iron beds, hydrothermal veins, and pegmatites, Cornwall (England), Erzberg (Austria, a whole mountain of it), Mont Saint-Hilaire, Brazil.
On value, the database notes: common; lustrous curved crystal groups modestly collectible.
Traditional associations.
In energy work traditions, siderite is said to steady the will and support practical persistence.
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.
Brown rhombohedra with gently curved, saddle-like faces and pearly sheen, calcite's heavyset iron cousin, half again denser and slow to fizz.
Not faked; routinely mislabeled among brown carbonates, curved faces plus heft plus weak fizz is the sorting key.
| Looks like | How to tell them apart |
|---|---|
| Calcite | Calcite is colorless-white, light, and fizzes furiously in cold vinegar; siderite barely reacts unless powdered or warmed |
| Dolomite/ankerite | Same curved-face family; siderite is darkest brown and heaviest, they grade into each other |
| Sphalerite | Resinous luster, sulfur-scented streak, six-direction cleavage |
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 siderite.
| Test | What you should see |
|---|---|
| By eye | Noticeably heavy for a carbonate (SG ~3.9) |
| Acid check | Weak fizz only when powdered or in warm acid |
| Magnet check | Turns magnetic after heating in a flame (converts toward magnetite) |
If these checks leave you unsure, that is what appraisers are for. Do not file, heat or soak a piece to find out.
Care & handling.
At 3.5 to 4.5 on the Mohs scale this is a soft material. Keep it as a display piece, store it apart from harder stones, and skip daily-wear settings.
It breaks with perfect rhombohedral cleavage; uneven fracture, which is worth knowing before you set it or drop it.
Most damage happens during cleaning, not wearing. Avoid steamers, ultrasonic tanks and anything from under the sink.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is FeCO3. Crystal system: Trigonal. Specific gravity: 3.96. Streak: White to pale yellow-brown. Cleavage and fracture: Perfect rhombohedral cleavage; uneven fracture.
How it reaches the market
Sedimentary iron beds, hydrothermal veins, and pegmatites, Cornwall (England), Erzberg (Austria, a whole mountain of it), Mont Saint-Hilaire, Brazil.
What it gets confused with
Calcite: Calcite is colorless-white, light, and fizzes furiously in cold vinegar; siderite barely reacts unless powdered or warmed. Dolomite/ankerite: Same curved-face family; siderite is darkest brown and heaviest, they grade into each other. Sphalerite: Resinous luster, sulfur-scented streak, six-direction cleavage.
What it is worth
Common; lustrous curved crystal groups modestly collectible.
Where buyers get caught
Not faked; routinely mislabeled among brown carbonates, curved faces plus heft plus weak fizz is the sorting key.
One more thing
Austria's Erzberg, 'the Ore Mountain', is essentially one giant siderite deposit mined since Roman times, complete with a legend that a water spirit traded the iron mountain for its freedom.
An education-first guide.
There is nothing to buy on this page. Siderite is not part of our catalogue, and the guide is here anyway because somebody searching the name deserves a straight answer.
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