sedimentary

Agate (translucent pebble)

Silicon dioxide (SiO2) - Microcrystalline Quartz

Hardness: 6.5-7 on Mohs scale; Color: Pale pinkish-brown, translucent; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64.

Hardness: 6
Color: Pale pinkish-brown, translucent
Luster: Waxy to vitreous

Identified May 19, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed as a secondary mineral in the cavities (vesicles) of volcanic rocks or sedimentary deposits. Silica-rich groundwater precipitates over long geological periods to fill these voids, often creating concentric banding, though small eroded pebbles may appear consistent in color.

Uses & applications

Used primarily in lapidary work for making jewelry (cabochons), ornamental carvings, and historical precision tools like mortar and pestles due to its hardness.

Geological facts

Agate is named after the Achates River in Sicily where it was first discovered. Translucent pebbles like this one are often polished by fluvial (river) or glacial action over thousands of years.

Field identification & locations

Identify in the field by its translucency (glows when backlit), waxy texture when wet, and lack of visible crystal faces. Common in gravel beds, beaches, and volcanic terrains. To test, check if it can scratch glass.

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