mineral
Polished agate geode slice
Banded chalcedony (microcrystalline quartz, SiO₂) with central quartz druse
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; gray, white, tan, and translucent concentric banding with a vitreous-to-waxy luster. The cavity is lined with tiny quartz crystals; quartz has trigonal crystal structure and no cleavage, with conchoidal fracture and specific gravity about 2.6.
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Physical properties
Hardness 6.5–7 Mohs; gray, white, tan, and translucent concentric banding with a vitreous-to-waxy luster. The cavity is lined with tiny quartz crystals; quartz has trigonal crystal structure and no cleavage, with conchoidal fracture and specific gravity about 2.6.
Formation & geological history
Forms as silica-rich groundwater fills vesicles in volcanic lava, depositing chalcedony in successive bands and later quartz crystals in the hollow. Most specimens are Cenozoic to Mesozoic, commonly derived from volcanic provinces.
Uses & applications
Used as a decorative polished slice, paperweight, or jewelry component; fine examples are popular affordable collectibles. Common commercial material comes from Brazil, Uruguay, Mexico, India, and the United States.
Geological facts
The visible concentric zones record changing chemistry and silica availability during cavity filling. Its central opening is a natural vug, not a hole drilled through the stone.
Field identification & locations
Identify agate by translucent layered bands, curved outlines, and waxy polish; quartz druse sparkles under magnification. Check edges for natural band continuity and avoid confusing dyed material with naturally colored agate.
