sedimentary

Banded Chert

Cryptocrystalline Quartz (SiO2)

Hardness: 6.5-7 on the Mohs scale. Color: Often black, grey, brown, or red with distinct layering (banding). Luster: Dull to waxy. Crystal Structure: Cryptocrystalline (microscopic crystals). Cleavage: None, exhibits conchoidal (shell-like) fracture. Specific Gravity: 2.5-2.8.

Hardness: 6
Luster: Dull to waxy

Identified March 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation of silica-rich organic remains such as radiolarian or diatom shells, or through the chemical precipitation of silica in marine environments. The banding represents fluctuations in sediment deposition over millions of years, commonly found in formations ranging from the Precambrian to more recent eras.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Modern uses include construction aggregate, gemstones for cabochons/lapidary work, and as a material for 'fire-starting' when struck against steel.

Geological facts

Banded chert is often associated with Banded Iron Formations (BIFs), which provide crucial evidence of the 'Great Oxygenation Event' in Earth's history. Some ancient cherts contain the oldest microfossils known to science.

Field identification & locations

Identify it in the field by its hardness (it will scratch glass), its distinctive smooth conchoidal fracture, and it lacks reaction to acid. It is commonly found in sedimentary basins, riverbeds, and as nodules in limestone or chalk. Collectors look for high-contrast banding and intense colors.