sedimentary

Chert (Nodule)

Microcrystalline Silica (SiO2)

Hardness: 6.5-7 on Mohs scale. Color: Gray, brown, or black with a waxy to dull luster. Crystal structure: Cryptocrystalline. It features a characteristic conchoidal fracture (curved, shell-like breaks) and is very dense.

Hardness: 6
Color: Gray, brown, or black with a waxy to dull luster

Identified June 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from seawater or the replacement of limestone/chalk by silica-rich fluids. It often occurs as nodules in sedimentary rock layers like limestone, dating from various geological eras from the Precambrian to the present.

Uses & applications

Historically used for stone tools and arrowheads due to its sharp edges when fractured. Currently used in road construction as aggregate, for decorative landscaping, and occasionally for flint-knapping enthusiasts.

Geological facts

Chert is so hard it can scratch glass and steel. When struck against steel, it can produce sparks, which is why a variety of it (flint) was famously used in early firearms and fire-starting kits.

Field identification & locations

Identify in the field by its smoothness, waxy luster, and sharp, curved edges where broken. It is commonly found in limestone outcrops or as rounded pebbles in stream beds. Collectors should look for unique colors or fossil inclusions.