sedimentary

Chert (Chalky/Gray Variety)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Gray with tan/brown weathered cortex; Luster: Dull to waxy; Structure: Cryptocrystalline (no visible crystals); Cleavage: None (displays conchoidal fracture); Specific Gravity: 2.5-2.8

Hardness: 6
Color: Gray with tan/brown weathered cortex
Luster: Dull to waxy

Identified March 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from groundwater within limestone or chalk beds. Most specimens date from the Phanerozoic eon.

Uses & applications

Historically used for stone tools (knapping), arrowheads, and fire-starting. Currently used as road aggregate, in landscaping, and occasionally as semi-precious gemstones when colorful (Jasper/Agate).

Geological facts

Chert is so hard it can scratch steel. It was one of the most vital materials for prehistoric humans because it breaks with razor-sharp edges, a property known as conchoidal fracture.

Field identification & locations

Identify by its waxy texture and shell-like (conchoidal) curved breaks. It is commonly found in nodules within limestone outcrops or as rounded pebbles in riverbeds and beaches globally.

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