Diatomite, also called diatomaceous earth, is a soft, siliceous sedimentary rock made of the fossilized silica remains of diatoms, microscopic single-celled algae that settled in lakes and seas. It crumbles to a fine white to off-white powder and has a low density because it is so porous. Marine deposits are worked in the Sisquoc Formation near Lompoc, California, and freshwater lake deposits occur in Nevada, Oregon, Washington and California, usually associated with volcanic rock. It is used to filter water, beer and wine and as an insecticide.

sedimentary

Diatomite

Also: Diatomaceous Earth, Kieselgur

No image

Is this diatomite?

3-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Read the texture
Look at a fresh break with a hand lens. Texture: Soft, porous rock that crumbles to a fine powder. Grain size: typically 10 to 200 micrometres.
2
Pick out the minerals
Composition: Fossilized diatom remains: 80 to 90 percent silica, mainly amorphous (opal), with minor clay and iron oxide. Identify the two or three most abundant grains.
3
Match the color range
Compare against the expected color range: white.

All properties

Colors: White
Uses: Industrial
Host rock: Lake and marine sediments; lake deposits are almost always associated with volcanic rock
Texture: Soft, porous rock that crumbles to a fine powder
Grain size: typically 10 to 200 micrometres
Composition: Fossilized diatom remains: 80 to 90 percent silica, mainly amorphous (opal), with minor clay and iron oxide

Where rockhounds find diatomite

Classic worldwide localities

Sisquoc Formation near Lompoc, California (marine)
Nevada, Oregon, Washington and California (freshwater lake deposits)
Maryland and Virginia (marine)
Fur and Mors, Denmark (moler)

Field-hunting tip

Where it forms: Lake and marine sediments; lake deposits are almost always associated with volcanic rock.

Common questions

Where is diatomite found?+

Notable localities include Sisquoc Formation near Lompoc, California (marine); Nevada, Oregon, Washington and California (freshwater lake deposits); Maryland and Virginia (marine); Fur and Mors, Denmark (moler).

Is diatomite safe to handle?+

Diatomite contains crystalline silica dust. Amorphous silica has low toxicity, but prolonged inhalation causes lung changes. Heat-treated diatomite, such as pool filter grades, contains crystalline silica.

What kind of rock does diatomite form in?+

Where it forms: Lake and marine sediments; lake deposits are almost always associated with volcanic rock. Working float back to the host body is the standard way to chase a fresh occurrence.

What is diatomite used for?+

Diatomite is used in industrial.

Find diatomite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.