sedimentary
Agate Nodule
Silicon Dioxide (Quartz/Chalcedony), SiO2
Hardness: 6.5-7 on the Mohs scale. Colors range from white, grey, brown to translucent varieties. Luster is waxy to vitreous. Crystal structure is microcrystalline (trigonal). Cleavage is none, fracture is conchoidal. Specific gravity is 2.58-2.64.
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 on the Mohs scale. Colors range from white, grey, brown to translucent varieties. Luster is waxy to vitreous. Crystal structure is microcrystalline (trigonal). Cleavage is none, fracture is conchoidal. Specific gravity is 2.58-2.64.
Formation & geological history
Formed by the deposition of silica from groundwater solutions in cavities of igneous and sedimentary rocks, typically over millions of years.
Uses & applications
Used in ornamental objects, jewelry, lapidary arts, and industrial knife-edge bearings or mortars and pestles.
Geological facts
Agates are often found in ancient volcanic vesicles or gas cavities. The concentric banding records successive pulses of silica-rich fluids entering the cavity.
Field identification & locations
Identify by its conchoidal fracture, hardness (scratches glass), waxy luster, and characteristic concentric banding when cut and polished. Found worldwide.
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