sedimentary

Chert (Iron-stained)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Cream, tan, and reddish-brown due to iron oxidation; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Cream, tan, and reddish-brown due to iron oxidation
Luster: Waxy to vitreous

Identified March 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or inorganic precipitation from silica-rich fluids within sedimentary rock layers, often occurring as nodules in limestone or chalk.

Uses & applications

Historically used for stone tools (arrowheads and scrapers) due to its sharp edges; modernly used in road construction, as an abrasive, and occasionally in lapidary work for cabochons.

Geological facts

Chert is virtually indestructible by weathering, which is why it is often found as loose stones in stream beds long after the surrounding limestone has eroded away. The red/orange staining is caused by 'limonite' or 'hematite' iron inclusions.

Field identification & locations

Identify in the field by its conchoidal (shell-like) fracture pattern and its ability to scratch glass easily. It will not react with acid, unlike the limestone it is often found in. Common worldwide in sedimentary basins.

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