mineral

Chalcedony (Agate/Chert)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale. Color: Translucent to opaque, varying from white, grey, brown, to tan. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline (microscopic). Cleavage: None (conchoidal fracture). Specific Gravity: 2.6.

Hardness: 6
Luster: Waxy to vitreous

Identified July 1, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the deposition of silica from groundwater in cavities of volcanic or sedimentary rocks. Common in hydrothermal environments and as nodules in limestone or volcanic ash beds.

Uses & applications

Used primarily for lapidary work (jewelry, cabochons), ornamental carvings, and historically for stone tools due to its sharp conchoidal fracture.

Geological facts

Chalcedony is a broad term that includes agate, jasper, and onyx. It was one of the first materials used by early humans to create arrowheads and scrapers because it breaks into predictably sharp edges.

Field identification & locations

Identify in the field by its waxy appearance, smooth conchoidal (shell-like) fracture surfaces, and the fact that it cannot be scratched by a steel knife. Frequently found in riverbeds or desert pavements in volcanic regions.

Other mineral specimens

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Schist

Metamorphic

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metamorphic

Gypsum (variety Selenite or Alabaster)

Calcium sulfate dihydrate (CaSO4·2H2O)

Mineral

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock