sedimentary

Chert (Hagstone/Gasket Rock variant)

Chert (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Reddish-brown, cream, and grey; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Reddish-brown, cream, and grey
Luster: Waxy to dull

Identified March 23, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from seawater or the replacement of limestone/chalk by silica-rich groundwater. The distinct holes (hagstone features) are typically formed by marine boring organisms like sponges or mollusks, or through mechanical water erosion and the dissolving of softer localized inclusions over millions of years.

Uses & applications

Historically used for stone tools and arrowheads. Currently used in construction aggregate, as landscaping stone, and as amulets or talismans in folk traditions (hagstones).

Geological facts

Rocks with naturally occurring holes are known as 'Hagstones' or 'Odin Stones.' According to European folklore, looking through the hole allows the viewer to see into the spirit realm or ward off evil spells and nightmares.

Field identification & locations

Identify in the field by its superior hardness (cannot be scratched by a steel pocketknife) and smooth, waxy texture compared to limestone. Found globally in riverbeds, beaches, and sedimentary outcrops. Collectors look for unique banding and naturally bored holes.