mineral

Chalcedony (Agate/Chert)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Blue-grey to tan/brown; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64

Hardness: 6
Color: Blue-grey to tan/brown
Luster: Waxy to vitreous

Identified July 15, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Blue-grey to tan/brown; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64

Formation & geological history

Formed as a chemical precipitate from silica-rich hydrothermal fluids or groundwater, often filling cavities (vesicles) in volcanic rocks or within sedimentary limestone nodules.

Uses & applications

Commonly used in jewelry for cabs and beads, as ornamental stone carvings, and historically for stone tools due to its conchoidal fracture which creates sharp edges.

Geological facts

Chalcedony is a cryptocrystalline form of silica, meaning its crystals are so small they can only be seen under high magnification. It is the base mineral for agate, jasper, and onyx.

Field identification & locations

Identify by its waxy luster, translucency, and shell-like (conchoidal) fracture surfaces. Commonly found in gravel beds, desert pavements, and volcanic regions worldwide.