mineral

Polished fortification agate (banded agate)

Microcrystalline quartz, SiO₂ (chalcedony with iron-oxide inclusions)

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; translucent brown, tan, and cream concentric banding; waxy to vitreous luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. The teardrop piece is polished and possibly drilled or shaped for jewelry.

Identified August 30, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rock, depositing successive layers of chalcedony and quartz over long periods, commonly during the Cenozoic or older volcanic episodes. Iron and manganese oxides create the reddish-brown coloration.

Uses & applications

Used for cabochons, pendants, beads, carvings, ornaments, and lapidary displays; agate is also used for small industrial wear-resistant components. Commonly found in volcanic provinces worldwide, including Brazil, Uruguay, Mexico, India, Madagascar, and the United States.

Geological facts

The sharply concentric, fortification-like lines are characteristic of fortification agate. It can resemble petrified wood, but agate generally shows continuous curved bands rather than cellular wood structure.

Field identification & locations

Identify it by its very fine parallel or angular bands, silica-like hardness, cool feel, and inability to scratch with a steel blade easily. Typical polished specimens sell for $5–$30; unusually vivid, large, or rare patterns may reach $50–$200.