mineral

Chalcedony (Common Opal or Agate variant)

Silicon Dioxide (SiO₂)

Hardness: 6.5-7 (Mohs scale); Color: Translucent milky white to grey; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline (Trigonal/Monoclinic); Cleavage: None; Specific Gravity: 2.58–2.64

Hardness: 6
Color: Translucent milky white to grey
Luster: Waxy to vitreous

Identified March 25, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the deposition of silica from water in volcanic cavities, sedimentary rocks, or hydrothermal veins. This process can happen over millions of years at relatively low temperatures.

Uses & applications

Commonly used in jewelry (cabochons and beads), decorative carvings, and as a semi-precious gemstone for collectors. Also used as an industrial abrasive in some forms.

Geological facts

Chalcedony is a cryptocrystalline form of silica, meaning its crystals are too small to be seen without extreme magnification. It has been used since the Bronze Age for seals and rings because it does not stick to hot wax.

Field identification & locations

Identify in the field by its waxy luster, conchoidal (shell-like) fracture, and inability to be scratched by a steel knife. Common in river beds and volcanic regions like Oregon, India, and Brazil.

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