sedimentary
Chalcedony (Agate/Chert)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: Translucent white to tan/yellowish, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.58-2.64
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: Translucent white to tan/yellowish, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.58-2.64
Formation & geological history
Formed from the deposition of silica from water solutions in volcanic cavities or sedimentary layers. It typically grows at low temperatures and can be found in a wide variety of geological ages, often as nodules or vein-filling material.
Uses & applications
Used primarily in lapidary arts for jewelry, beads, and carvings. Industrially, it has historical use as a hard-wearing surface and for making tools due to its sharp conchoidal fracture.
Geological facts
Agate is a variety of chalcedony that is specifically characterized by banding. The translucent nature and waxy luster of this specimen are hallmarks of the silica family. It is nearly identical in chemistry to quartz but differs in its microscopic crystal structure.
Field identification & locations
Identify in the field by its waxy appearance and the way it breaks into smooth, curved surfaces (like glass). It cannot be scratched by a steel knife. It is commonly found in riverbeds, desert pavements, and volcanic regions.
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