Mineral/Sedimentary
Agate Nodule (partially vuggy Quartz)
Silicon Dioxide (SiO2)
Hardness: 6.5–7 (Mohs scale); Color: White, gray, and brown; Luster: Vitreous to waxy; Crystal structure: Trigonal/Hexagonal (cryptocrystalline); Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- White, gray, and brown
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 6.5–7 (Mohs scale); Color: White, gray, and brown; Luster: Vitreous to waxy; Crystal structure: Trigonal/Hexagonal (cryptocrystalline); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed as secondary deposits of silica in cavities of volcanic rocks or sedimentary formations. Silica-rich fluids percolate through voids over millions of years, often leaving a hollow 'vug' or pocket of crystals.
Uses & applications
Used primarily for lapidary work, jewelry (cabochons), as collector decorative pieces, and historically for mortars and pestles due to its hardness.
Geological facts
Agate is a variety of chalcedony. This specific specimen shows a 'vuggy' center where macrocrystalline quartz has grown into an open pocket, which is common in geodes and thunder eggs.
Field identification & locations
Identify by its hardness (scratches glass), waxy luster on broken surfaces, and specific gravity. Look for it in riverbeds, volcanic ash beds, or desert environments. This specimen is a small rough nodule.
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