sedimentary

Banded chert (banded microcrystalline quartz)

Chert, chiefly microcrystalline quartz (SiO₂), with iron-oxide and clay impurities

AI-generated identification · may be incorrect

Typically Mohs 6.5–7, gray to brown/greenish-black, waxy to dull luster, very fine-grained and nonfoliated; lacks visible cleavage and breaks with a conchoidal fracture. The specimen shows strongly contorted, rhythmic silica-rich bands, possibly stromatolitic or agate-like layering.

Identified August 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when silica-rich marine or groundwater deposits lithify and recrystallize, commonly replacing carbonate sediments; banding may reflect changing chemistry or microbial mats. Exact age cannot be determined from the photograph; chert ranges from Precambrian to modern.

Uses & applications

Used historically for tools and fire-starting, and today as lapidary material, tumbled stone, ornaments, and decorative cabochons; generally unsuitable as a major construction aggregate when attractive banding is preserved.

Geological facts

Its dense microcrystalline quartz structure makes it resistant to weathering and capable of producing a sharp edge. The curved bands are natural mineral layering, not wood grain, though similar specimens are sometimes sold as stromatolite or banded jasper.

Field identification & locations

Identify by its hardness, dense smooth surface, lack of obvious grains, and conchoidal fracture; a steel file generally scratches it weakly or not at all. Common in sedimentary formations worldwide, including the Great Lakes, western United States, Australia, and parts of Europe.