Identification guide · Carbonates
How to Identify Siderite
Siderite is usually yellowish brown, brown, and tan with a vitreous luster and is translucent. It has a Mohs hardness of 4. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Its streak is white. It is most often confused with Magnesite, Quartz, and Calcite; the tests below separate them.
Siderite identification checklist
| Color | Yellowish brown, Brown, Tan, Gray | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Translucent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 4 | A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect rhombohedral | |
| Fracture | Uneven to conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 3.96 | Heavier than it looks (specific gravity about 3.96). |
| Crystal system | Trigonal |
Extra tests for Siderite
Acid test. Fizzes in vinegar or dilute hydrochloric acid because of its carbonate content.
Glass check. Siderite shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Siderite look-alikes and how to tell them apart
Both sit near Mohs 4, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Siderite vs Magnesite →
Quartz is harder (Mohs 7) than Siderite (Mohs 4), so Quartz will scratch Siderite and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Siderite vs Quartz →
Siderite is harder (Mohs 4) than Calcite (Mohs 3), so Siderite will scratch Calcite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Siderite vs Calcite →
Both sit near Mohs 4, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Field tips
- Look for honey-brown to tan rhombohedral crystals, often with curved saddle-shaped faces.
- Siderite has perfect rhombohedral cleavage and is notably heavy for a carbonate.
- Unlike calcite, siderite reacts only weakly with cold dilute acid and may need warm acid to fizz.
Siderite identification FAQ
How can you tell if a rock is Siderite?
Check the color (yellowish brown, brown, tan, and gray), the vitreous luster, a Mohs hardness of 4 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Siderite?
Siderite has a Mohs hardness of 4. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not.
What is the difference between Siderite and Magnesite?
Siderite (Mohs 4, vitreous luster) and Magnesite (Mohs 4, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Siderite vs Magnesite comparison for a side-by-side table.
Is Siderite worth anything?
Entry-level Siderite specimens sell for around $5, typical collector pieces about $45, and premium examples $350 or more depending on size, quality and locality.
What color is Siderite?
Siderite is most often yellowish brown, brown, tan, and gray. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
