sedimentary

Agate Thunderegg

Silicon dioxide (SiO2) within rhyolitic host rock

Hardness of 7 on Mohs scale. Displays concentric bands of chalcedony with a waxy luster and cryptocrystalline structure.

Identified November 14, 2026More sedimentary →

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Physical properties

Hardness of 7 on Mohs scale. Displays concentric bands of chalcedony with a waxy luster and cryptocrystalline structure.

Formed in gas cavities of volcanic ash beds or lava flows filled by silica-rich hydrothermal fluids over millions of years.

Used primarily in lapidary arts, gemstone carving, and ornamental jewelry making.

Thundereggs are the state rock of Oregon and form spherical nodule shapes in volcanic deposits.

Found in desert regions and volcanic deposits worldwide; identified by cutting or cracking open rough exterior nodules.

Fossil-bearing concretion, possibly containing a fossil shell or other organic inclusion

Calcareous concretion; exact fossil and host-rock composition are not identifiable from the image

Sandstone or Siltstone

Arenite (if sandstone)

Fossiliferous Sedimentary Rock

Sedimentary limestone with invertebrate fossil imprint

Shale

Fissile Sedimentary Rock

Sandstone

Arenite

Shale

Clastic sedimentary rock