Mineral within Sedimentary Rock

Chalcedony in Limestone Host Rock

Silicon Dioxide (SiO2) within Calcium Carbonate (CaCO3)

Hardness: 7 (inner crystal), 3 (outer rock). Color: Grey to translucent white. Luster: Waxy or dull. Structure: Microcrystalline (Quartz) and Massive. Cleavage: None/Conchoidal.

Hardness: 7 (inner crystal), 3 (outer rock)
Color: Grey to translucent white
Luster: Waxy or dull

Identified March 23, 2026More mineral within sedimentary rock →

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Physical properties

Hardness: 7 (inner crystal), 3 (outer rock). Color: Grey to translucent white. Luster: Waxy or dull. Structure: Microcrystalline (Quartz) and Massive. Cleavage: None/Conchoidal.

Formation & geological history

Formed through secondary mineralization where silica-rich fluids precipitate into cavities or vugs within sedimentary limestone formations, often during the Paleozoic era.

Uses & applications

Used primarily for decorative geological specimens, historical flint-knapping for tools, or as a source of high-purity silica in industrial abrasive applications.

Geological facts

This specimen appears to be a small 'vug' or geode-like cavity. Chalcedony is technically a fibrous form of cryptocrystalline quartz that is too small to see with the naked eye.

Field identification & locations

Identify in the field by checking for the hardness difference; the inner grey crystals will scratch glass, while the outer grey rock will not. Commonly found in riverbeds or limestone quarries.

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