sedimentary

Grey chert or siliceous limestone nodule

Microcrystalline quartz (chalcedony), likely with calcite and clay impurities

AI-generated identification · may be incorrect

Typically light-to-dark gray, dull to waxy luster, very fine-grained and massive, with conchoidal fracture; Mohs hardness about 6.5–7 if chert, or 3 if limestone. The image shows a hard-looking gray nodule with irregular lighter weathering and possible fossil or cavity textures.

Identified August 18, 2026More sedimentary →

Explore Grey chert or siliceous limestone nodule in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Most likely formed as a sedimentary replacement or cementation body in marine limestone, commonly during the Paleozoic–Mesozoic, though the exact age cannot be determined visually. Silica-rich groundwater replaced carbonate sediment or filled cavities.

Uses & applications

Chert is used historically for tools and fire-starting, while attractive pieces are collected or cut for cabochons and décor; limestone varieties are used as aggregate and cement feedstock. This specimen appears primarily collectible rather than commercial.

Geological facts

Chert can preserve fossils and may show sharp conchoidal edges when freshly broken. A steel file or glass-scratch test can help distinguish chert from softer limestone, though testing may damage the specimen.

Field identification & locations

Common in limestone formations worldwide, including the United States, Europe, and parts of Asia. Look for a glassy fracture and inability to scratch it with a steel knife; fizzing in dilute hydrochloric acid indicates exposed calcite rather than pure chert.