Identification guide · Sedimentary Rocks
How to Identify Diatomite
Diatomite is usually white, cream, and light gray with a dull luster and is opaque. It has a Mohs hardness of 1.5. A fingernail (Mohs 2.5) scratches it easily. Its streak is white. It is most often confused with Limestone, Oolite, and Caliche; the tests below separate them.
Diatomite identification checklist
| Color | White, Cream, Light gray, Tan | 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 | 1.5 | A fingernail (Mohs 2.5) scratches it easily. |
| Streak | White | 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 chalky | |
| Specific gravity | 1 | Noticeably light in the hand for its size (specific gravity about 1). |
Diatomite look-alikes and how to tell them apart
Limestone is harder (Mohs 3) than Diatomite (Mohs 1.5), so Limestone will scratch Diatomite and not the other way round. A fingernail (Mohs 2.5) scratches it easily. Diatomite vs Limestone →
Oolite is harder (Mohs 3) than Diatomite (Mohs 1.5), so Oolite will scratch Diatomite and not the other way round. A fingernail (Mohs 2.5) scratches it easily. Diatomite vs Oolite →
Caliche is harder (Mohs 3) than Diatomite (Mohs 1.5), so Caliche will scratch Diatomite and not the other way round. A fingernail (Mohs 2.5) scratches it easily. Diatomite vs Caliche →
Marl is harder (Mohs 2.5) than Diatomite (Mohs 1.5), so Marl will scratch Diatomite and not the other way round. A fingernail (Mohs 2.5) scratches it easily.
Field tips
- Look for a very light, soft, chalky rock.
- Diatomite may be white, cream, tan, or pale gray.
- It can feel porous or absorbent compared with limestone.
- Avoid breathing dust from powdered or crumbly material.
Diatomite identification FAQ
How can you tell if a rock is Diatomite?
Check the color (white, cream, light gray, and tan), the dull luster, a Mohs hardness of 1.5 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Diatomite?
Diatomite has a Mohs hardness of 1.5. A fingernail (Mohs 2.5) scratches it easily.
What is the difference between Diatomite and Limestone?
Diatomite (Mohs 1.5, dull luster) and Limestone (Mohs 3, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Diatomite vs Limestone comparison for a side-by-side table.
Is Diatomite worth anything?
Entry-level Diatomite 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 Diatomite?
Diatomite is most often white, cream, light gray, and tan. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
