sedimentary

Chert (Jasper/Flint variety)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale. Color: Dark gray to brownish-black. Luster: Waxy to vitreous (especially when wet). Crystal structure: Microcrystalline (trigonal system). Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.

Hardness: 6
Color: Dark gray to brownish-black
Luster: Waxy to vitreous (especially when wet)

Identified May 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms through the precipitation of silica from groundwater or the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) in marine environments. It can occur in almost any geological age, commonly found in limestone and chalk beds.

Uses & applications

Historically used for stone tools and weapons due to its sharp conchoidal fracture. Today used as an abrasive, in decorative stone collections, and occasionally as a low-cost gemstone.

Geological facts

Chert is so hard it can strike sparks from steel, which led to its historically vital use as 'flint' in firearms and fire-starting kits. It is one of the most durable rocks on Earth's surface.

Field identification & locations

Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and its shell-like (conchoidal) fracture pattern. It often looks like a river pebble but maintains sharp edges when broken.

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Sandstone or Siltstone

Arenite (if sandstone)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone Grain

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Shale or Slaty Mudstone

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