sedimentary
Crinoid Marble (Fossilized Limestone)
Crinoidal Limestone (comprising Calcite/Aragonite CaCO3)
Hardness: 3-4 (Mohs scale); Color: Pale grey, tan, or greenish-grey matrix with white/cream fossil discs; Luster: Dull to sub-vitreous; Structure: Bioclastic sedimentary; Cleavage: Perfect rhombohedral in calcite crystals.
- Hardness
- 3-4 (Mohs scale)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 3-4 (Mohs scale); Color: Pale grey, tan, or greenish-grey matrix with white/cream fossil discs; Luster: Dull to sub-vitreous; Structure: Bioclastic sedimentary; Cleavage: Perfect rhombohedral in calcite crystals.
Formation & geological history
Formed during the Paleozoic era (approx. 300-500 million years ago) from the accumulated skeletal segments (ossicles) of crinoids, marine animals related to starfish, which were buried in lime mud on the seafloor.
Uses & applications
Used primarily as decorative stone, for lapidary work (making marbles or cabochons), and as educational fossils for collectors.
Geological facts
Crinoids are also known as 'sea lilies' because they resemble plants, though they are actually animals. These 'marbles' are often produced by tumbling or polishing naturally rounded pebbles found in riverbeds or beaches.
Field identification & locations
Identify by looking for small, repetitive circular or donut-shaped 'cheerio' patterns which are the cross-sections of crinoid stems. Common in the UK, Midwest USA, and Morocco.
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