mineral

Blue chalcedony or common opal

Cryptocrystalline silica, SiO₂; likely pale blue chalcedony, possibly opal-A

AI-generated identification · may be incorrect

Pale blue, translucent, waxy-to-glassy luster with a smoothly tumbled surface; hardness about 6.5–7 for chalcedony or 5.5–6.5 for opal, with no visible cleavage and conchoidal fracture. The image alone cannot reliably distinguish the two.

Identified September 13, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities and fractures in volcanic rocks; opal is hydrated silica, while chalcedony is microcrystalline quartz. Deposits range from relatively young volcanic-hosted material to Paleozoic and older sedimentary occurrences.

Uses & applications

Used as a lapidary stone, cabochon, carving material, and inexpensive jewelry or decorative specimen; blue chalcedony can be collectible when strongly colored and translucent.

Geological facts

Blue chalcedony’s color commonly comes from microscopic inclusions or trace elements, whereas precious opal shows play-of-color, absent here. A refractive-index and specific-gravity test is the best confirmation.

Field identification & locations

Common sources include Turkey, Namibia, Madagascar, Indonesia, Brazil, and the western United States. Identify chalcedony by its hardness, waxy appearance, and lack of opal fire; avoid destructive tests on a small polished piece.