sedimentary

Limestone (River Pebble)

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Off-white, light gray, or tan; Luster: Dull to earthy; Crystal structure: Hexagonal (as calcite/aragonite particles); Cleavage: Perfect rhombohedral in calcite crystals, usually not visible in massive form; Specific gravity: 2.3–2.7.

Hardness: 3 on Mohs scale
Color: Off-white, light gray, or tan
Luster: Dull to earthy

Identified March 29, 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 through the accumulation of shell, coral, algal, and fecal debris in marine environments. This specific specimen has been further shaped by fluvial (river) erosion, rounding it into a smooth pebble over thousands of years.

Uses & applications

Major source of lime for cement, production of glass, used as a building stone (dimension stone), and as a soil conditioner in agriculture to neutralize acidity.

Geological facts

Limestone accounts for about 10% of the total volume of all sedimentary rocks. Most of the world's caves are formed in limestone because it is soluble in weak organic acids.

Field identification & locations

Can be identified in the field by its softness (it can be scratched by a copper coin or steel knife) and its reaction to dilute hydrochloric acid (it will fizz or effervesce). It is commonly found in riverbeds, coastlines, and quarries globally.

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