Sedimentary Rocks · common
Marl
Marl is an earthy sedimentary rock or sediment made of mixed clay and calcium carbonate. It often looks gray, cream, tan, or soft and chalky.
Also known as: Calcareous mudstone, Marlstone.
Marl properties at a glance
| Type | Sedimentary Rocks |
|---|---|
| Chemical formula | Calcium carbonate and clay-rich sediment |
| Mohs hardness | 2.5 |
| Luster | dull |
| Streak | White to light gray |
| Cleavage | Variable by mineral grains |
| Fracture | Earthy to uneven |
| Specific gravity | 2.4 |
| Transparency | opaque |
| Common colors | Gray, Cream, White, Tan, Greenish gray |
| Rarity | common |
Geology & background
Forms from mixtures of carbonate mud and clay in lakes, shallow seas, or low-energy sedimentary environments.
Marl has been used in agriculture and helps geologists interpret lakes, shallow marine settings, and carbonate mud environments.
How to identify Marl
- 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.
Full Marl identification guide: checklist, home tests and look-alikes →
Marl vs similar stones
Not sure it isn't something else? Side-by-side comparisons with the stones most often confused with Marl.
Where Marl is found
Marl occurs in many sedimentary settings worldwide; exact locality should be verified from field notes, maps, or specimen provenance.
How much is Marl worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Marl is usually valued for teaching, field context, locality, texture, fossils, color, or unusual structures rather than high gem value.
Common uses
- Field identification
- Agricultural soil amendment
- Cement raw material
- Education
Interesting facts
- Marl sits between clay-rich mudrock and carbonate-rich limestone.
- It may fizz weakly with acid because of carbonate content.
- Marl has historically been used to improve agricultural soils.
Care & safety
Marl can be soft, porous, crumbly, or weakly cemented depending on grain size, cement, and weathering.
Avoid inhaling dust. Do not cut, grind, or polish unknown sedimentary rocks without eye protection and a properly rated respirator.
Marl FAQ
What is Marl?
Marl is an earthy sedimentary rock or sediment made of mixed clay and calcium carbonate. It often looks gray, cream, tan, or soft and chalky.
How much is Marl worth?
Entry-level Marl specimens sell for around $1, a typical collector specimen for about $10, and premium pieces for $75 or more. Marl is usually valued for teaching, field context, locality, texture, fossils, color, or unusual structures rather than high gem value.
How hard is Marl?
Marl has a Mohs hardness of 2.5. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
What is Marl used for?
Marl is used for field identification, agricultural soil amendment, cement raw material, and education.
