sedimentary

Chert / Flint Nodules in Sandstone

Microcrystalline Quartz (SiO2) within Clastic Sedimentary Rock

AI-generated identification · may be incorrect

Hardness: 7 (silica nodules); Color: tan to reddish-brown matrix with grey/translucent nodules; Luster: waxy or vitreous nodules with dull/earthy matrix; Cleavage: none (conchoidal fracture in nodules).

Hardness: 7 (silica nodules)
Color: tan to reddish-brown matrix with grey/translucent nodules
Luster: waxy or vitreous nodules with dull/earthy matrix

Identified March 23, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed via chemical precipitation of silica within porous sedimentary layers, often in marine or groundwater environments. These nodes frequently occur in limestones or sandstones of various geological ages, notably from the Paleozoic to the Mesozoic eras.

Uses & applications

Used historically for stone tools and fire-starting (flint). Currently used as aggregate in construction, or if colorful, for lapidary work and river rock landscaping.

Geological facts

If the silica nodule is rounded and occurs within a lighter matrix, it is often called a 'bird's eye' or a 'concretion'. Flint was the primary material for the Neolithic 'Stone Age' tools due to its predictable conchoidal fracturing.

Field identification & locations

Identify in the field by looking for smooth, glassy or waxy circular 'windows' within a rougher, gritty host rock. Common in gravel beds, stream deposits, and outcrops of sedimentary formations like the Appalachian or Ozark plateaus.