Chalcedony (Agate/Quartz)

Physical Properties

Hardness: 6.5-7 (Mohs); Color: translucent white to brownish-grey; Luster: waxy to vitreous; Structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific Gravity: 2.6

Formation & Geological History

Formed from the deposition of silica from groundwater in cavities of igneous rocks, particularly volcanic lavas, or in sedimentary environments. Common in Cenozoic deposits.

Uses & Applications

Commonly used for lapidary work, jewelry (beads and cabochons), ornamental carvings, and as a collector's specimen.

Geological Facts

The specimen on the right appears to be a polished 'nodule' or river-rock chalcedony, while the one on the left shows a natural sugary crystalline surface typical of a geode fragment.

Field Identification & Locations

Identify in the field by its hardness (scratches glass), waxy luster on broken surfaces, and lack of visible crystal grains. Often found in riverbeds or volcanic ash beds.

Identified on: 5/13/2026

Mode: Standard