mineral

Chalcedony (Agate Pebble)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale. Color: Greyish-blue interior with a rusty brown iron-stained exterior. Luster: Waxy to vitreous. Crystal Structure: Trigonally microcrystalline. Cleavage: None. Fracture: Conchoidal.

Hardness: 6
Luster: Waxy to vitreous

Identified July 14, 2026More mineral →

Explore Chalcedony (Agate Pebble) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through the deposition of silica from groundwater in cavities of volcanic or sedimentary rocks. The weathered exterior suggests it was transported by water (alluvial) or exposed to prolonged oxidation.

Uses & applications

Used primarily in lapidary work for cabochons, beads, and carvings. Also used in industrial applications requiring high hardness and chemical resistance, such as mortars and pestles.

Geological facts

Chalcedony is a cryptocrystalline form of silica. This specific specimen shows a classic 'nodule' structure where the weathered outer rind protects the translucent interior. The blue-grey tint is common in varieties often called 'Blue Lace' or 'Cloud' chalcedony.

Field identification & locations

Identify in the field by its waxy luster, conchoidal (shell-like) fracture, and inability to be scratched by a steel knife. It is often found in riverbeds or areas with ancient volcanic activity.