sedimentary

Fossiliferous Limestone (Crinoidal Limestone)

Calcium Carbonate with Crinoid Stem Fossils (CaCO3)

AI-generated identification · may be incorrect

Hardness: 3 (Mohs scale), Color: Dirty white to light grey, Luster: Dull/Earthy, Crystal Structure: Hexagonal (calcite matrix), Cleavage: Perfect rhombohedral in calcite, Specific Gravity: 2.71

Identified June 12, 2026More sedimentary →

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Physical properties

Hardness: 3 (Mohs scale), Color: Dirty white to light grey, Luster: Dull/Earthy, Crystal Structure: Hexagonal (calcite matrix), Cleavage: Perfect rhombohedral in calcite, Specific Gravity: 2.71

Formation & geological history

Formed in warm, shallow marine environments from the accumulation of skeletal fragments of marine organisms, specifically crinoids (sea lilies). Most common in Paleozoic strata, particularly Carboniferous to Mississippian periods.

Uses & applications

Primarily used in construction (aggregate, cement), as building stone, in landscaping, and as educational scientific specimens.

Geological facts

The circular or donut-shaped holes/inclusions are 'columnals' or sections of the stalk of a crinoid, a marine animal related to starfish. These are some of the most common fossils found in North American limestone.

Field identification & locations

Identify by looking for 'cheerio' or 'button' shaped rings on the surface. Commonly found in riverbeds, quarries, and rocky outcrops throughout the Midwest US and UK. Reacts with a fizzing sound when exposed to weak acid (vinegar).