sedimentary
Chalcedony (Agate)
Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs), Color: Translucent yellowish-tan, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microcrystalline), Cleavage: None, Conchoidal fracture
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs), Color: Translucent yellowish-tan, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microcrystalline), Cleavage: None, Conchoidal fracture
Formation & geological history
Formed via precipitation of silica-rich groundwater in cavities of igneous or sedimentary rocks. This specimen appears to be a river-worn or water-eroded nodule.
Uses & applications
Used for lapidary work, jewelry, decorative carvings, and as a semi-precious gemstone due to its durability.
Geological facts
Chalcedony is a cryptocrystalline form of silica, meaning its crystals are too small to be seen with a regular microscope. It has been used for making seal stones and jewelry since the Bronze Age.
Field identification & locations
Identify in the field by its translucency (light passes through edges), waxy texture when wet, and lack of visible crystal grains. Often found in gravel beds or volcanic regions.
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