sedimentary
Agate (Chert/Chalcedony)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown, tan, and cream with translucent areas; Luster: Waxy to vitreous; Crystal structure: Microcrystalline/Cryptocrystalline (Trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.60-2.65.
- Hardness
- 6
- Color
- Reddish-brown, tan, and cream with translucent areas
- Luster
- Waxy to vitreous
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown, tan, and cream with translucent areas; Luster: Waxy to vitreous; Crystal structure: Microcrystalline/Cryptocrystalline (Trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.60-2.65.
Formation & geological history
Formed in cavities of volcanic or sedimentary rocks where silica-rich groundwater precipitates over long periods. Often found in riverbeds or glacial tills as weathered nodules.
Uses & applications
Used primarily in lapidary arts, jewelry making, ornamental carvings, and historical tool making (knapping). Industrial uses include precision mortars and pestles.
Geological facts
Agates are essentially banded chalcedony. The specimen shown exhibits a pitted surface typical of Lake Superior Agates or similar glacial drift agates where softer matrix has eroded away.
Field identification & locations
Identify in the field by looking for a waxy luster, translucency when held to light, and characteristic surface pitting or 'gas bubbles'. Commonly found in gravel pits and shorelines across the Upper Midwest USA.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite
sedimentary