sedimentary

Chert Nodule

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, brownish-tan to bluish-gray; Luster: Waxy to dull, earthy; Crystal Structure: Cryptocrystalline/microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.58-2.64

Hardness: 7 on Mohs scale
Color: Gray, brownish-tan to bluish-gray
Luster: Waxy to dull, earthy

Identified July 27, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in sedimentary environments, often through biochemical or inorganic precipitation of silica within limestone or chalk beds. Over millions of years, silica replaces organic matter or carbonates to form hard nodules.

Uses & applications

Historically used for making stone tools, flints, and arrowheads due to its sharp conchoidal fracture. Today used as crushed stone, in landscape gardening, and as a decorative geological specimen.

Geological facts

Chert has been used by ancient humans for hundreds of thousands of years to produce fire-starting sparks and extremely sharp cutting tools.

Field identification & locations

Identified in the field by its extreme hardness (scratches glass), waxy luster, smooth conchoidal fracture surfaces, and occurrence as rounded nodules in limestone or chalk matrix. Commonly found in limestone quarries and stream beds.