mineral
Blue-gray chalcedony (banded agate fragment)
Chalcedony, microcrystalline quartz (SiO₂), likely agate variety
AI-generated identification · may be incorrectMohs hardness 6.5–7; waxy to vitreous luster; translucent blue-gray color with faint curved banding and a rough quartz-rich rind. Quartz has a trigonal crystal structure, but chalcedony is composed of microscopic intergrowths and lacks visible cleavage; specific gravity is about 2.58–2.64.
Identified More mineral →
Explore Blue-gray chalcedony (banded agate fragment) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Mohs hardness 6.5–7; waxy to vitreous luster; translucent blue-gray color with faint curved banding and a rough quartz-rich rind. Quartz has a trigonal crystal structure, but chalcedony is composed of microscopic intergrowths and lacks visible cleavage; specific gravity is about 2.58–2.64.
Formation & geological history
Forms when silica-rich groundwater fills cavities and fractures in volcanic rocks, depositing layers of microcrystalline quartz over time. Most examples formed during the Cenozoic or older volcanic episodes, depending on locality.
Uses & applications
Used for cabochons, beads, carvings, lapidary pieces, and ornamental stone; attractive banded material has modest jewelry value.
Geological facts
The blue-gray color commonly results from microscopic inclusions, light scattering, or trace minerals rather than a separate blue mineral. Agate banding records repeated changes in silica-rich fluids during cavity filling.
Field identification & locations
Common in volcanic terrains worldwide, including Brazil, Uruguay, Mexico, India, Madagascar, and the western United States. Identify it by its waxy translucence, conchoidal fracture, hardness, and curved internal bands; a steel file should not easily scratch it.
