mineral
Gray chalcedony (possibly common opal)
Microcrystalline quartz, SiO₂; potentially hydrated silica, SiO₂·nH₂O
AI-generated identification · may be incorrectTranslucent blue-gray to gray, waxy to vitreous luster, conchoidal fracture, no visible cleavage, and Mohs hardness about 6.5–7 for chalcedony (5.5–6.5 if opal). The rounded white-gray patches may be botryoidal or inclusion-rich zones.
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Physical properties
Translucent blue-gray to gray, waxy to vitreous luster, conchoidal fracture, no visible cleavage, and Mohs hardness about 6.5–7 for chalcedony (5.5–6.5 if opal). The rounded white-gray patches may be botryoidal or inclusion-rich zones.
Formation & geological history
Likely formed when silica-rich groundwater filled cavities or fractures in volcanic rock, depositing chalcedony or opal in layers and nodules. Chalcedony can form from the Cenozoic to present in hydrothermal and weathering environments.
Uses & applications
Used as a lapidary material for cabochons, carvings, beads, and decorative objects; tougher, well-colored examples are collectible. It is not normally used as a major construction aggregate.
Geological facts
Chalcedony is quartz built from microscopic crystals, so it lacks the large crystal faces of quartz. The specimen’s translucency and conchoidal edges favor chalcedony, though exact identification requires hardness, refractive-index, or specific-gravity testing.
Field identification & locations
Common in volcanic regions, especially Brazil, Uruguay, Mexico, the western United States, India, and Turkey; identify by its waxy translucency, lack of cleavage, and glass-like conchoidal fracture. A steel file should scratch opal but generally not chalcedony.
