sedimentary
Chalcedony (Agate/Chert variety)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: tan, brownish-yellow, translucent; Luster: waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: none (conchoidal fracture); Specific Gravity: 2.58-2.64.
- Hardness
- 6
- Color
- tan, brownish-yellow, translucent
- Luster
- waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: tan, brownish-yellow, translucent; Luster: waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: none (conchoidal fracture); Specific Gravity: 2.58-2.64.
Formation & geological history
Forms by the deposition of silica from groundwater in cavities of igneous rocks or as nodules in sedimentary rocks like limestone. Common in volcanic regions and ancient seabed deposits.
Uses & applications
Used in jewelry as gemstones, for ornamental carvings, and historically for making stone tools due to its sharp conchoidal fracture.
Geological facts
Chalcedony is a cryptocrystalline form of silica. Agate is specifically the banded variety, while translucent, non-banded versions like this are often just called chalcedony or chert.
Field identification & locations
Identify in the field by its translucency when held to light and its smooth, waxy feel. Often found in riverbeds or gravel deposits. Common in the Western United States and Brazil.
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