sedimentary

Banded Chert or Agate

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs); Color: Dark brown to black with tan/brown bands; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Hardness: 6
Color: Dark brown to black with tan/brown bands
Luster: Waxy to vitreous

Identified May 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich fluids or microorganisms (like radiolaria or diatoms) in sedimentary environments, or as secondary deposits in volcanic cavities. This specimen appears water-worn, likely from a river or beach deposit.

Uses & applications

Used historically for tool-making (arrowheads/scrapers) due to conchoidal fracture. Modern uses include tumbling for gravel, jewelry cabochons, and decorative landscaping pebbles.

Geological facts

Chert was one of the first materials used by early humans for tools because it breaks with a razor-sharp edge. Dark varieties of microcrystalline quartz are often referred to as 'flint' or 'jasper' depending on the color and location.

Field identification & locations

Identify in the field by its hardness (it should scratch glass), waxy feel when smooth, and the presence of parallel layers or bands. Common in gravel pits, riverbeds, and sedimentary rock formations like limestone.