sedimentary
Chert / Flint (with Crinoid Stem Molds)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: tan to brown with pitted circular indentations, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan to brown with pitted circular indentations, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed as a chemical precipitate in marine environments or through the replacement of limestone. This specimen contains 'molds' where fossilized crinoid stems were once embedded before dissolving. Most common in Paleozoic aged strata (roughly 250-500 million years old).
Uses & applications
Historically used for stone tools and fire-starting (flint). Currently used in road construction as aggregate, for lapidary work like tumbling, and as decorative garden stones.
Geological facts
The circular pits on this specimen are known as 'crinoid molds.' Crinoids, or sea lilies, are animals related to starfish. While the organic stem is gone, the impression left in the hard chert remains permanently.
Field identification & locations
Identify in the field by its superior hardness (cannot be scratched by steel) and the presence of small circular or ring-shaped pits. Common in the American Midwest and Appalachian regions. Collectors look for high fossil density.
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