sedimentary

Chert (Nodular or Massive Variety)

Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Brown, tan, grey, or greenish-grey; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Brown, tan, grey, or greenish-grey
Luster: Waxy to dull

Identified March 26, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Brown, tan, grey, or greenish-grey; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by the chemical replacement of limestone/chalk by silica-rich fluids. Often formed in deep-sea environments during the Paleozoic or Mesozoic eras.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp fracture edges. Modernly used in highway construction as aggregate and occasionally as a gemstone (jasper/agate varieties) or for aquarium landscaping.

Geological facts

Chert is so hard it can create sparks when struck against steel, which made it a vital survival tool for early humans. Its extreme durability allows it to survive long-term erosion that destroys surrounding rock.

Field identification & locations

Identify in the field by its very high hardness (it will scratch glass) and its signature conchoidal (shell-like) fracture patterns. It is commonly found in limestone outcrops or as weathered river cobbles.