mineral

Chalcedony Nodule (Commonly known as Agate or Chert)

Microcrystalline Quartz (SiO2)

Hardness: 6.5–7 on Mohs scale; Color: Translucent gray to brownish with yellowish-white outer rind; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.58–2.64

Hardness: 6
Color: Translucent gray to brownish with yellowish-white outer rind
Luster: Waxy to vitreous

Identified March 5, 2026More mineral →

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Physical properties

Formation & geological history

Formed by the deposition of silica from groundwater in cavities of volcanic or sedimentary rocks. This specimen likely originated as a replacement mineral or a pocket filling within a host rock millions of years ago, often during the Cenozoic or Mesozoic eras.

Uses & applications

Used primarily for jewelry (cabochons), lapidary art, and ornamental carvings. Historically used for making tools like scrapers and arrowheads due to its conchoidal fracture which creates sharp edges.

Geological facts

Chalcedony is technically a fibrous form of silica. When it exhibits banding, it is called Agate. The weathered 'skin' or rind on this specimen is known as the 'cortex,' which is often opaque and softer than the interior mineral.

Field identification & locations

Identify in the field by its waxy texture, translucency when held to a light source, and its ability to scratch glass. Commonly found in riverbeds, gravel pits, and volcanic regions globally. Collectors should look for smoothness against more porous host rocks.

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