sedimentary
Dark Agate / Chalcedony
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Dark brown, grey, translucent amber; Luster: Waxy to vitreous; Crystal structure: Trigonal/Monoclinic (microcrystalline); Cleavage: None (Conchoidal fracture).
- Hardness
- 6
- Color
- Dark brown, grey, translucent amber
- Luster
- Waxy to vitreous
Identified More sedimentary →
Explore Dark Agate / Chalcedony in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6.5-7 Mohs; Color: Dark brown, grey, translucent amber; Luster: Waxy to vitreous; Crystal structure: Trigonal/Monoclinic (microcrystalline); Cleavage: None (Conchoidal fracture).
Formation & geological history
Formed by the deposition of silica from groundwater in cavities of volcanic igneous rocks or sedimentary formations. Its age can range from recent to hundreds of millions of years based on the host rock environment.
Uses & applications
Used largely for lapidary work, jewelry making (cabochons), as decorative garden stones, or in scientific instruments like mortars and pestles due to its hardness.
Geological facts
Agate is a variety of chalcedony characterized by its fineness of grain and brightness of color. Even though this specimen looks dark, holding it up to a strong light source may reveal internal banding or transparency.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (it will scratch glass and steel), and smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds or volcanic regions.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Brown Sandstone (River Rock)
Arenite
sedimentary
Mudstone
Argillite / Siliciclastic sedimentary rock
sedimentary
Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary