sedimentary

Chert (and Agate/Quartz in Geode-like Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Brown, gray, white, and translucent; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Brown, gray, white, and translucent
Luster: Waxy to vitreous

Identified July 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from groundwater or hydrothermal fluids within a host rock (likely limestone or volcanic ash). This specimen shows a nodular structure where silica has replaced original material or filled a cavity.

Uses & applications

Historically used for stone tools (flintknapping); currently used as a decorative garden stone, for lapidary work (polishing), and as a collector's specimen.

Geological facts

Chert is remarkably durable and resistant to chemical weathering. When it contains translucent, banded layers, it is often referred to as agate. This specific piece looks like a silica-filled nodule or a 'thunder egg' fragment.

Field identification & locations

Identify in the field by its waxy luster on broken surfaces and its ability to scratch glass. It often breaks with sharp, shell-like (conchoidal) edges. Found worldwide in sedimentary basins and volcanic deposits.

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