sedimentary
Crinoid Fossil in Chert
Siliceous limestone with Crinoidea fossil (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: tan to honey-yellow, Luster: waxy to vitreous, Crystal Structure: microcrystalline quartz (chert), Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: tan to honey-yellow, Luster: waxy to vitreous, Crystal Structure: microcrystalline quartz (chert), Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed in Paleozoic era marine environments (mostly Mississippian and Pennsylvanian) where silica replaced limestone. The hole is a mold left behind when the internal stem segment of a crinoid dissolved away.
Uses & applications
Educational purposes, lapidary use, fossil collecting, and occasionally used as decorative garden stones.
Geological facts
The square or round holes in these rocks are often called 'fairy stones' or 'Indian beads'. They are actually the internal voids of ancient 'sea lilies' which are echinoderms related to modern starfish.
Field identification & locations
Identify in the field by looking for small, geometric (circular or square) holes in water-worn river stones. Common in the Midwest USA and river beds worldwide.
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