sedimentary
Crinoid Fossil Limestone
Crinoidal Limestone (Calcite / CaCO3)
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
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Physical properties
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
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.
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