sedimentary
Chert (Tan/Yellow variety)
Microcrystalline quartz (SiO2)
Hardness: 7 on Mohs scale; Color: tan, buff, or light brown; Luster: waxy to dull; Crystal structure: cryptocrystalline (microscopic grains); Cleavage: none, exhibits conchoidal fracture; Specific gravity: approx. 2.65.
- Hardness
- 7 on Mohs scale
- Color
- tan, buff, or light brown
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: tan, buff, or light brown; Luster: waxy to dull; Crystal structure: cryptocrystalline (microscopic grains); Cleavage: none, exhibits conchoidal fracture; Specific gravity: approx. 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical precipitation of silica in groundwater and deep oceanic environments. Often occurs as nodules within limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in construction aggregate, road surfacing, and as a semi-precious stone for tumbling and jewelry if colorful.
Geological facts
Chert is a chemical sedimentary rock. It is so hard that it can scratch steel and glass. It is often found as nodules or layers within sedimentary sequences and has been used by humans for hundreds of thousands of years for tool-making.
Field identification & locations
Identify in the field by its hardness (will scratch a knife blade), shell-like (conchoidal) fracture pattern, and waxy luster on fresh surfaces. Often found in riverbeds or within limestone outcrops.
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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