Identification guide · Sedimentary Rocks
How to Identify Marl
Marl is usually gray, cream, and white with a dull luster and is opaque. It has a Mohs hardness of 2.5. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Its streak is white to light gray. It is most often confused with Dolomite, Limestone, and Dolostone; the tests below separate them.
Marl identification checklist
| Color | Gray, Cream, White, Tan, Greenish gray | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Dull | Compare a fresh surface with glass and metal; this luster is softer than either. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 2.5 | A copper coin (Mohs 3.5) scratches it, but a fingernail does not. |
| Streak | White to light gray | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Variable by mineral grains | |
| Fracture | Earthy to uneven | |
| Specific gravity | 2.4 | Ordinary rock-like heft (specific gravity about 2.4), similar to quartz. |
Extra tests for Marl
Acid test. Fizzes in vinegar or dilute hydrochloric acid because of its carbonate content.
Marl look-alikes and how to tell them apart
Dolomite is harder (Mohs 3.5–4) than Marl (Mohs 2.5), so Dolomite will scratch Marl and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Marl vs Dolomite →
Both sit near Mohs 2.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Marl vs Limestone →
Dolostone is harder (Mohs 3.5) than Marl (Mohs 2.5), so Dolostone will scratch Marl and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Marl vs Dolostone →
Siltstone is harder (Mohs 3.5) than Marl (Mohs 2.5), so Siltstone will scratch Marl and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
Field tips
- Look for soft, earthy gray, cream, tan, or greenish material.
- Marl may react weakly with dilute acid because it contains carbonate.
- It is usually finer and more clay-rich than typical limestone.
- Fresh surfaces can look dull, powdery, or chalky.
Marl identification FAQ
How can you tell if a rock is Marl?
Check the color (gray, cream, white, and tan), the dull luster, a Mohs hardness of 2.5 and a white to light gray streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Marl?
Marl has a Mohs hardness of 2.5. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
What is the difference between Marl and Dolomite?
Marl (Mohs 2.5, dull luster) and Dolomite (Mohs 3.5–4, vitreous to dull luster) look alike, but hardness, streak and heft usually separate them. See the full Marl vs Dolomite comparison for a side-by-side table.
Is Marl worth anything?
Entry-level Marl specimens sell for around $1, typical collector pieces about $10, and premium examples $75 or more depending on size, quality and locality.
What color is Marl?
Marl is most often gray, cream, white, and tan. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
