sedimentary

Chert (within Limestone)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Cream, tan, or yellowish-brown waxy center surrounded by lighter matrix; Luster: Waxy to vitreous (glassy) center, dull outer matrix; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7

Hardness: 7 (Mohs scale)
Luster: Waxy to vitreous (glassy) center, dull outer matrix

Identified May 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the replacement of limestone by silica-rich fluids or the accumulation of siliceous organisms (like radiolarians) on the sea floor. This typically occurs in marine environments across many geological periods, notably the Paleozoic and Mesozoic eras.

Uses & applications

Historically used for stone tools and arrowheads due to its sharp edges when broken. Today it is used in road construction as aggregate, as a component in ceramics, and occasionally for lapidary work.

Geological facts

Chert is so hard it can scratch glass. It was one of the first materials ever used by humans to create sharp tools. The variation of chert known as 'Flint' is famous for its ability to produce sparks when struck against steel.

Field identification & locations

Identify in the field by its hardness (it should scratch a pocket knife blade) and its smooth, shell-like (conchoidal) fracture pattern. It often appears as nodules or layers trapped within softer limestone or chalk formations. Common globally, especially in the central US and the UK.