Chert Pebble/Nodule

Hardness: 6.5-7 (Mohs scale), Color: Varies widely, often grey, brown, black, green, or red, Luster: Dull to waxy, Crystal Structure: Cryptocrystalline (crystals too small to be seen without high magnification), Cleavage: None, exhibits conchoidal fracture, Specific Gravity: 2.58-2.65

Chert forms from the accumulation of silica-rich skeletal remains of microscopic marine organisms (like radiolarians and diatoms) or by the diagenetic alteration of a silica-rich gel. It can also form through the replacement of other minerals, such as limestone, by silica-rich waters. It typically forms within sedimentary environments, often in marine settings, and can be found disseminated in bedrock or as discrete nodules or layers within other sedimentary rocks (like limestone). Geological age varies depending on the specific deposit, ranging from Precambrian to Cenozoic.

Historically, chert was highly valued for making tools due to its hardness and conchoidal fracture, which allowed for the creation of sharp edges for blades, arrowheads, and scrapers. In modern times, it's sometimes used as an aggregate in construction materials, and attractive varieties can be cut and polished for ornamental purposes or cabochon jewelry.

Flint is a dark grey to black variety of chert, specifically formed within chalk or limestone. The conchoidal fracture of chert is what made it ideal for knapping by early humans, producing very sharp tools. This property is similar to that of obsidian (a volcanic glass).

In the field, chert is often identified by its hardness (it scratches glass and cannot be scratched by a steel knife), its characteristic conchoidal fracture (producing smooth, curved surfaces with sharp edges when broken), and its often dull to waxy luster. It's commonly found in sedimentary rock sequences, particularly limestone and chalk beds, and can be found as pebbles in riverbeds and on beaches, transported from its original bedrock source. The image shows a weathered chert pebble with a somewhat rough, lighter-colored outer layer or exposed internal texture, typical of rock nodules that have been tumbled in water.