Chalcedony (Smoky Quartz or Agate nodule fragment)

Physical Properties

Hardness: 6.5-7 on Mohs scale. Color: Translucent tan to smoky brownish-gray. Luster: Waxy to vitreous. Crystal Structure: Hexagonal/Trigonal (cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.65.

Formation & Geological History

Formed through the slow deposition of silica-rich groundwater in cavities of igneous or sedimentary rocks. This specific specimen appears to be a small water-worn nodule or fragment of chalcedony.

Uses & Applications

Used primarily in lapidary arts, jewelry (cabochons), and as a collector's specimen. Industrial uses include abrasives and precision mechanical bearings due to its hardness.

Geological Facts

Chalcedony is a cryptocrystalline form of silica, meaning its crystals are too small to be seen without extreme magnification. The smoky color is often caused by natural radiation or trace impurities of aluminum.

Field Identification & Locations

Identify in the field by its waxy luster, translucency, and shell-like (conchoidal) fractures. It will scratch glass but cannot be scratched by a steel knife. Commonly found in riverbeds and volcanic regions.

Identified on: 4/26/2026

Mode: Standard