sedimentary

Gray Limestone Fragment

Limestone (CaCO3)

Hardness: 3 on Mohs scale; Color: Gray to brownish-gray; Luster: Dull/Earthy; Structure: Fine-grained, microcrystalline; Cleavage: None/Irregular fracture; Specific Gravity: 2.5-2.7

Hardness: 3 on Mohs scale
Color: Gray to brownish-gray
Luster: Dull/Earthy

Identified July 6, 2026More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale; Color: Gray to brownish-gray; Luster: Dull/Earthy; Structure: Fine-grained, microcrystalline; Cleavage: None/Irregular fracture; Specific Gravity: 2.5-2.7

Formation & geological history

Formed by the accumulation of shell, coral, algal, and fecal debris in clear, warm, shallow marine waters. This specific grain likely eroded from a larger rock bed through physical weathering.

Uses & applications

Larger quantities are used in construction, road base, cement manufacturing, and agriculture. Small individual pebbles are primarily of interest to hobbyist collectors or as garden gravel.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is highly reactive to acids and will fizz when in contact with dilute hydrochloric acid.

Field identification & locations

Small gray pebbles like this are ubiquitous in gravel driveways, riverbeds, and coastal regions. Field identification can be confirmed with a simple acid test or by its relatively soft surface that can be scratched by a steel nail.