sedimentary

Agate (Geode/Nodule)

Chalcedony (cryptocrystalline silica, SiO2)

Hardness: 6.5-7 (Mohs scale), Color: Brown, tan, and grey with possible internal banding, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microscopic), Cleavage: None/Conchoidal fracture, Specific Gravity: 2.60-2.65

Hardness: 6

Identified July 1, 2026More sedimentary →

Explore Agate (Geode/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

Formed primarily as nodules in volcanic rocks or certain sedimentary rocks where silica-rich groundwater fills cavities left by gas bubbles or dissolved organic material. Most range from 20 to 100 million years old, depending on the host formation.

Uses & applications

Used extensively in jewelry (cabochons), interior decor, abrasive tools specifically mortar and pestle sets, and as a popular collector's item for lapidary work.

Geological facts

Agate was named by the Greek philosopher Theophrastus who discovered the stone along the shore line of the river Achates in Sicily. No two natural agates are identical in their internal banding patterns.

Field identification & locations

Identified in the field by its characteristic 'orange peel' or bumpy exterior texture and conchoidal (shell-like) fracture edges. Commonly found in riverbeds, desert pavements, and volcanic ash deposits in places like Brazil, Uruguay, and the US Northwest.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary