sedimentary

Crinoid Fossil Limestone

Crinoidal Limestone (Calcite / CaCO3)

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs scale), Color: light gray with tan inclusions, Luster: dull to earthy, Crystal Structure: Trigonal (microcrystalline matrix), Cleavage: rhombohedral in calcite portions, Specific Gravity: 2.7

Identified June 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of skeletal fragments of crinoids (sea lilies) on the ocean floor, typically during the Paleozoic era (approx. 250-500 million years ago). The matrix is lime mud that has lithified over millions of years.

Uses & applications

Primarily used for geological study, education, and as decorative garden stones or common collector items. Large deposits may be used as building stone or in the production of cement.

Geological facts

Crinoids are actually animals (echinoderms) related to starfish and sea urchins, though they look like plants. The ring-shaped depressions or 'beads' in this rock are the cross-sections of their segmented stems.

Field identification & locations

Identify in the field by looking for circular or star-shaped impressions and fossilized 'beads' in gray or tan limestone. Commonly found in riverbeds and limestone quarries in the Midwest United States and UK.