mineral

Agate Nodule (Chalcedony)

Silicon Dioxide (SiO2) - Cryptocrystalline Quartz

Hardness: 6.5-7 on Mohs scale. Color: Translucent grayish-white, bluish-gray, yellowish tints with distinct concentric white banding. Luster: Waxy to vitreous. Crystal structure: Trigonal (microcrystalline/cryptocrystalline). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.58-2.64.

Hardness: 6
Luster: Waxy to vitreous

Identified July 26, 2026More mineral →

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Physical properties

Formation & geological history

Forms primarily in cavities (vesicles) within volcanic rocks such as basalt or rhyolite. Silica-rich fluids percolate into these gas pockets over thousands to millions of years, depositing microcrystalline quartz layer by layer from the outer rim inward.

Uses & applications

Popular for lapidary work, jewelry making (cabochons, beads), decorative tumbled stones, and rock collecting. Historically used for carving ornamental objects and seals.

Geological facts

Agates are defined by their characteristic banding patterns formed by alternating layers of silica minerals (chalcedony and quartz). Small nodules like this are often nicknamed 'thunder eggs' or agate nodules depending on their host rock formation.

Field identification & locations

Identify by looking for translucent microcrystalline silica with characteristic concentric or wavy banding and a hard, waxy outer rind. Commonly found in basaltic gravel beds, riverbeds, and desert pavement in areas with ancient volcanic activity (e.g., Brazil, USA, Mexico, Madagascar).

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