sedimentary
Geode Nodules / Agate Chert
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs). Color: Dusty grey-brown exterior with iron staining. Luster: Dull waxy exterior. Crystal structure: Cryptocrystalline. Specific gravity: 2.6-2.65.
- Hardness
- 6
- Color
- Dusty grey-brown exterior with iron staining
- Luster
- Dull waxy exterior
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6.5-7 (Mohs). Color: Dusty grey-brown exterior with iron staining. Luster: Dull waxy exterior. Crystal structure: Cryptocrystalline. Specific gravity: 2.6-2.65.
Formation & geological history
Formed through the deposition of silica-rich groundwater within cavities of sedimentary or volcanic host rock. Often found in limestone beds where silica replaces calcite or fills voids.
Uses & applications
Used for lapidary work (when cut and polished), geological study, and as decorative pieces for collectors.
Geological facts
Geodes are often called 'thunder eggs' when they are solid or filled with agate. This specimen's cauliflower-like exterior is a classic sign of a silica nodule that may contain crystals inside.
Field identification & locations
Look for rounded, bumpy, or 'cauliflower' textured stones that are unusually heavy or light for their size. Common in the Midwest USA (Keokuk region) and Brazil.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
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
sedimentary