sedimentary

Chert (Chalcedony)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, white, and translucent with waxy luster; Crystal Structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 (Mohs scale)
Color: Gray, tan, white, and translucent with waxy luster

Identified April 7, 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 chemical precipitation of silica in marine or groundwater environments. It often occurs as nodules in limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads). Modern uses include gemstones (when colorful, known as jasper or agate), construction aggregate, and as a material for precision mechanical parts due to its hardness.

Geological facts

Chert has been used by humans for hundreds of thousands of years because it breaks with incredibly sharp edges. It is often referred to as 'flint' when found in chalk deposits and 'jasper' when it is pigmented red or yellow.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), waxy to dull luster, and conchoidal (shell-like) fracture patterns. Frequently found in riverbeds, glacial till, or eroding out of limestone outcrops.