mineral
Banded agate and chalcedony rough
Microcrystalline quartz (SiO₂), chiefly chalcedony with agate banding; some pieces may include jasper or brecciated quartz
AI-generated identification · may be incorrectMohs hardness 6.5–7; waxy to dull luster, translucent gray, brown, red, and cream colors, with irregular/conchoidal fracture and occasional curved bands or druzy quartz. Specific gravity is about 2.58–2.65; crystals are microscopic quartz fibers.
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Physical properties
Mohs hardness 6.5–7; waxy to dull luster, translucent gray, brown, red, and cream colors, with irregular/conchoidal fracture and occasional curved bands or druzy quartz. Specific gravity is about 2.58–2.65; crystals are microscopic quartz fibers.
Formation & geological history
Forms when silica-rich groundwater fills cavities and fractures in volcanic rocks, depositing chalcedony and successive colored bands; associated jasper forms from iron-rich impurities. The specimens are likely Cenozoic or older volcanic-hosted material, but age requires locality data.
Uses & applications
Used for cabochons, beads, carvings, lapidary slabs, and decorative objects; crushed chalcedony is also used as silica aggregate. Rough material is mainly collected for pattern, color, and polishability.
Geological facts
Agate’s banding records repeated changes in fluid chemistry during cavity filling. Red and brown colors commonly result from iron oxides, while blue-gray areas may reflect finely fibrous chalcedony.
Field identification & locations
Identify by testing hardness against glass, observing waxy translucency, conchoidal fracture, and any concentric banding; it should not scratch easily with a steel knife. Common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the western United States.
