mineral

Polished black agate geode slice

Microcrystalline quartz (chalcedony), SiO₂, with possible iron-oxide matrix and druzy quartz

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; black, gray, cream, and rust-brown banding with vitreous to waxy luster. It has cryptocrystalline quartz, locally containing cavities lined with sparkling druzy crystals; no obvious cleavage, conchoidal fracture, and specific gravity about 2.6.

Identified August 17, 2026More mineral →

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Physical properties

Formation & geological history

Agate formed when silica-rich groundwater filled gas cavities in volcanic rock, depositing chalcedony in successive bands and sometimes quartz crystals in the center. The host volcanics are commonly Cenozoic to Mesozoic, depending on locality.

Uses & applications

Used as a decorative slab, bookend, paperweight, or collector specimen; agate is also used for cabochons and ornamental objects. Value depends on size, polish, pattern, and crystal development.

Geological facts

The contrasting brown rind is likely iron-stained host rock or outer agate. Dark coloration may be natural manganese/iron staining or enhancement, so dyeing cannot be ruled out visually.

Field identification & locations

Identify by curved concentric banding, glassy fracture, hardness, and inability to scratch with a steel knife; it should scratch glass. Common sources include Brazil, Uruguay, Mexico, Madagascar, and the western United States.