sedimentary

Chert

Chert (microcrystalline quartz, SiO2)

Hardness: 7 on Mohs scale; Color: Varies widely, often gray, brown, black, red, or white. The specimen appears to be a reddish-brown with some lighter inclusions. Luster: Dull to waxy; Crystal structure: Primarily cryptocrystalline or microcrystalline, meaning crystals are too small to be seen with the naked eye;…

Hardness: 7 on Mohs scale
Color: Varies widely, often gray, brown, black, red, or white
Luster: Dull to waxy

Identified November 8, 2025More sedimentary →

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Physical properties

Formation & geological history

Chert forms from the accumulation of silica-rich skeletal remains of diatoms, radiolarians, and sponge spicules in marine or lacustrine environments (biogenic chert), or through the replacement of existing rocks by silica-rich fluids (replacement chert). It can often form as nodules within limestone or dolomite. Its geological age can range from Precambrian to Cenozoic depending on the specific formation.

Uses & applications

Historically, chert was widely used by early humans for tools (arrowheads, spear points, knives) due to its hardness and ability to fracture into sharp edges. Today, it's used as an aggregate in construction, road building, and occasionally as a decorative stone. Some attractive varieties are polished for jewelry.

Geological facts

Chert is a very hard and durable rock that played a crucial role in human technological development. Varieties of chert include flint (dark gray to black, often found in chalk formations), jasper (red, yellow, brown, or green opaque), and radiolarite (formed from radiolarian skeletons). It's also known to be the primary component of novaculite, a fine-grained sedimentary rock used as a sharpening stone.

Field identification & locations

Chert can be identified in the field by its hardness (it will scratch glass), its conchoidal fracture, and its typically dull to waxy luster. It often occurs as nodules or beds within limestone or shale. Common locations include sedimentary basins worldwide where silica-rich organisms flourished or where silification occurred. Tips for collectors include looking for sharp, angular fragments with a smooth, curved break.