sedimentary

Limestone (River Pebble)

Calcium Carbonate (CaCO3)

AI-generated identification · may be incorrect

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 →

Explore Limestone (River Pebble) in the encyclopedia →

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.