mineral
Tumbled milky chalcedony (likely common opalite-like quartz)
Chalcedony, microcrystalline SiO₂; possibly pale common opal, hydrated SiO₂·nH₂O
AI-generated identification · may be incorrectHardness about 6.5–7 for chalcedony (5.5–6.5 if opal); translucent milky blue-white, waxy to vitreous luster, no visible crystals, and conchoidal fracture. The smooth rounded shape indicates tumbling and polishing.
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Physical properties
Hardness about 6.5–7 for chalcedony (5.5–6.5 if opal); translucent milky blue-white, waxy to vitreous luster, no visible crystals, and conchoidal fracture. The smooth rounded shape indicates tumbling and polishing.
Formation & geological history
Chalcedony forms when silica-rich groundwater fills cavities and fractures, commonly in volcanic rocks; opal forms from hydrated silica gel. Formation ages range from recent to millions of years, depending on the host deposit.
Uses & applications
Used as an inexpensive lapidary stone, cabochon, bead, carving, and decorative tumbled stone; similar material is also used for ornamental objects.
Geological facts
The blue cast may result from thin-section scattering, inclusions, or artificial lighting; strongly glowing blue material sold as “opalite” is often man-made glass, while natural opal is softer and may show play-of-color.
Field identification & locations
Check hardness against glass, inspect for bubbles (suggesting glass), and use a loupe for fibrous chalcedony texture or opal play-of-color. Common sources include Brazil, Uruguay, Mexico, Turkey, and the western United States.
