sedimentary
Quartz Nodule / Agate Geode
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale. Color: Milky white quartz core with yellowish-brown to dark iron oxide weathering crust. Luster: Waxy to vitreous. Crystal Structure: Trigonal/Microcrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale. Color: Milky white quartz core with yellowish-brown to dark iron oxide weathering crust. Luster: Waxy to vitreous. Crystal Structure: Trigonal/Microcrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
Formation & geological history
Formed as secondary mineral infillings within cavities (vesicles) of volcanic or sedimentary host rocks. Over time, silica-rich solutions deposit microcrystalline quartz and chalcedony layer by layer. The exterior matrix is often heavily weathered by iron oxide staining.
Uses & applications
Popular for lapidary work, tumbling, slabbing to reveal inner banding, or cracking open to check for hollow geode centers with interior quartz crystals. Used in ornamental carvings and rock collections.
Geological facts
Nodules like this often hide vibrant agate banding, chalcedony layers, or hollow geode centers containing quartz or amethyst crystals. The exterior iron oxide rind is created by long-term weathering in soil or gravel beds.
Field identification & locations
Look for rounded, heavy nodules with a bumpy or cauliflower-like texture and iron-stained exterior. Commonly found in riverbeds, desert gravels, and weathered volcanic areas (such as the Midwestern US, Oregon, and Brazil). Cutting with a lapidary saw is necessary to reveal internal beauty.
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