mineral

Banded Agate (specifically Sardonyx)

Silicon Dioxide (SiO2) - variety of Chalcedony/Quartz

Hardness: 6.5-7 on Mohs scale. Color: Alternating bands of black, white, and translucent brownish-yellow (sard). Luster: Vitreous to waxy. Crystal Structure: Trigonal (cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.60–2.65.

Hardness: 6
Luster: Vitreous to waxy

Identified March 25, 2026More mineral →

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Physical properties

Formation & geological history

Formed in volcanic cavities or cooling lava bubbles (vesicles). Silica-rich groundwater percolates through the rock, depositing layers of microcrystalline quartz over thousands of years. Many date back to the Cenozoic era, though formation can occur in any volcanic environment.

Uses & applications

Used in jewelry (cabochons), carved cameos, beads, and as decorative gemstones. Industrial uses include mortars and pestles for grinding chemicals due to its extreme hardness and chemical resistance.

Geological facts

Sardonyx has been used for over 4,000 years; Ancient Romans often wore it as amulets engraved with Mars to bring bravery in battle. It is the original birthstone for August.

Field identification & locations

Identify by looking for distinct, parallel, concentric, or wavy banding and a waxy surface texture. It is commonly found in Brazil, India, Uruguay, and the Lake Superior region of the U.S. Collectors look for high contrast between the colored bands.

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