Diatomite Identification Guide

What Diatomite Looks Like

Diatomite is a soft, lightweight, fine-grained siliceous sedimentary rock composed of the fossil silica shells (frustules) of diatoms. It is the lithified (hardened) equivalent of diatomaceous earth. It appears white, cream, buff, or pale gray, is chalky and porous, and is light enough to sometimes float briefly before soaking up water.

Key visual cues:

• Color: white, cream, light gray, sometimes pale tan.
• Texture: very fine, chalky, porous; can be cut or carved easily.
• Weight: unusually light for a rock.
• Luster: dull, earthy.

Step-by-Step Field-ID Checklist

1. Heft it. Diatomite is conspicuously light and porous.
2. Scratch test. It is soft and can be marked with a fingernail or knife; it powders to a fine chalky dust.
3. Tongue/absorbency test. It sticks slightly to the tongue and absorbs water rapidly.
4. Acid test. Apply dilute HCl — diatomite does NOT fizz (it is silica).
5. Loupe/microscope check for diatom frustules confirms identity.

Diagnostic Tests

• Hardness: soft as a rock (scratched by fingernail/knife); silica shells themselves are harder.
• Streak/feel: chalky white residue.
• Density: very low bulk density due to high porosity; often <1.5 g/cm³, much lighter than chalk or chert.
• Acid: inert to dilute HCl — distinguishes from carbonate chalk/limestone.
• Absorbency: high; sticks to tongue.

Common Look-Alikes and How to Tell Them Apart

• Chalk (limestone): chalk fizzes strongly in acid; diatomite does not. Decisive separator.
• Chert/flint: chert is the dense, hard (Mohs ~7) silica rock with conchoidal fracture; diatomite is soft, porous, and light despite both being silica.
• Volcanic tuff/ash: can look similar but is usually denser and lacks diatom frustules under the microscope.
• Kaolin/claystone: clay becomes plastic and sticky when wet; diatomite stays crumbly and gritty.
• Diatomaceous earth: same composition but unconsolidated/powdery; diatomite is the coherent rock.

Where It Is Found

Diatomite forms in lacustrine and marine basins where diatom-rich sediment accumulated and then lithified. Important deposits include Lompoc, California (one of the world's largest), plus Nevada, Oregon, and Washington; Denmark (moler), France, Spain, and Algeria. It is quarried for filtration media, lightweight aggregate, abrasives, and insulation, and is found as soft pale beds within sedimentary sequences.