sedimentary

Oolite (Ooids)

Oolitic Limestone (Principal mineral: Calcite, CaCO3)

Hardness: 3 (Mohs scale); Color: Typically white, gray, or tan; Luster: Dull to pearly; Structure: Spherical or subspherical grains (ooids) usually <2mm in diameter; Cleavage: Perfect rhombohedral in individual crystals but not visible in the aggregate shell.

Hardness: 3 (Mohs scale)
Color: Typically white, gray, or tan
Luster: Dull to pearly

Identified January 3, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed in warm, supersaturated, shallow marine waters. Ooids develop as concentric layers of calcium carbonate precipitate around a nucleus (like a sand grain or shell fragment) while being rolled by wave action. Most commonly found in strata from the Jurassic, Mississippian, and Holocene eras.

Uses & applications

Used as high-quality building stone (e.g., Portland stone), in the manufacture of lime and cement, as a flux in steel production, and sometimes as an aggregate in road construction.

Geological facts

The name comes from the Greek word 'oion' for egg, because the rock resembles fish roe. Famous examples include the 'Indiana Limestone' used in the Empire State Building and the Pentagon.

Field identification & locations

Field identification involves looking for small, egg-shaped grains that react with dilute hydrochloric acid (fizzing). They are commonly found in high-energy coastal environments like the Great Bahama Bank or the Great Salt Lake.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary