sedimentary

Chert (specifically Blue or Grey Chert/Flint)

Microcrystalline or cryptocrystalline Quartz (SiO2)

Hardness: 7 on the Mohs scale; Color: Blue-grey, slate, or charcoal with light streaks; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on the Mohs scale
Color: Blue-grey, slate, or charcoal with light streaks
Luster: Waxy to dull

Identified March 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the precipitation of silica from groundwater within limestone or chalk beds. Most specimens date from the Paleozoic or Mesozoic eras.

Uses & applications

Used historically for stone tools and weapons due to its sharp edges; industrially used as an abrasive or for road surfacing. In jewelry, it is often tumbled or cut into cabochons.

Geological facts

Because of its conchoidal fracture, it breaks into sharp edges like glass, making it the primary material for prehistoric arrowheads and spear points. It is also famous for its ability to create sparks when struck against steel.

Field identification & locations

Identify in the field by its smooth, waxy feel and shell-like (conchoidal) curved break patterns. Found worldwide, often in nodules within limestone or as river-worn pebbles. Collectors look for unique banding or 'denim' blue hues.

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone

Arenite (primarily SiO2)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

Arenite (SiO2)

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Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

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