Sedimentary Rock

Chert (varietal known as Jasper or Flint)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale. Color: Highly variable, commonly grey, brown, green, red, or black. The specimen appears dark grey to reddish-brown. Luster: Dull to waxy. Crystal Structure: Cryptocrystalline (microscopic crystals), no visible crystal structure. Fracture: Conchoidal (shell-like, with curved ripples).…

Hardness: 6
Color: Highly variable, commonly grey, brown, green, red, or black
Luster: Dull to waxy

Identified October 25, 2025More sedimentary rock →

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Physical properties

Formation & geological history

Chert typically forms in marine environments where silica-rich solutions precipitate, often replacing other minerals or filling voids in limestone. It can also form during diagenesis (changes to sediment after deposition) of silica-rich organic remains (like diatoms or radiolarians). Its geological age can vary widely, from Precambrian to Cenozoic, depending on the specific deposit. The presence of sand adhering to it suggests it was found in an environment where erosion and deposition occur (e.g., beach, riverbed).

Uses & applications

Historically, flint (a variety of chert) was crucial for making tools and weapons in the Stone Age due to its ability to fracture into sharp edges. Today, it can be used as an abrasive, in road construction as aggregate, or as a decorative stone. High-quality, colorful varieties (like jasper) are used in jewelry and ornamental carvings.

Geological facts

Chert is a very common sedimentary rock. Flint is a dark gray to black variety of chert, often found in chalk or limestone deposits, famous for its use in striking sparks for fire and early weaponry. The term 'chert' is often used interchangeably with 'flint,' especially for less pure or less dark varieties.

Field identification & locations

In the field, chert can be identified by its hardness (scratches glass), conchoidal fracture, and often waxy or dull luster. It will not react with acid (unlike limestone), indicating the absence of carbonates. It is commonly found in sedimentary rock formations, especially limestones and chalks, and as cobbles and pebbles in riverbeds and on beaches. The specimen's somewhat blocky, angular shape with a slightly rounded edge suggests it has been worn by water and might be a fragment of a larger chert nodule or bed.

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