mineral

Chalcedony (Common Opal variant)

Silicon dioxide (SiO2) with varying levels of hydration

Hardness: 6.5-7 (Mohs scale), Color: Translucent white to milky blue-grey, Luster: Waxy to vitreous, Crystal Structure: Cryptocrystalline/Amorphous, Cleavage: None (conchoidal fracture), Specific Gravity: 2.15-2.65

Hardness: 6

Identified May 21, 2026More mineral →

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Physical properties

Formation & geological history

Formed at low temperatures from silica-rich groundwater precipitated in the cavities of igneous rocks or through sedimentary replacement of fossils and organic matter.

Uses & applications

Used primarily in lapidary work, jewelry (cabochons), and as a healing stone in metaphysical communities. Lower grades are used for industrial abrasives.

Geological facts

Chalcedony was highly prized in the ancient world for making cylinder seals in Mesopotamia and signet rings in Rome because wax does not stick to it.

Field identification & locations

Identify by its waxy luster, conchoidal (shell-like) fracture, and lack of visible crystals. It is commonly found in basalt cavities (geodes) or as weathered nodules in desert regions like the American Southwest.

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