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Smithsonian National Museum of Natural History

Quartz (var. agate)

Photograph of bright red agate (136330) from the National Mineral Collection
Photo by Chip Clark. Click to zoom.
Catalog Number NMNH 136330-00
Locality Chihuahua, Mexico

Agate is a fine-grained variety of quartz known as chalcedony that shows concentric color banding. The colors in this agate are caused by microscopic inclusions of iron oxide minerals in the quartz. Agates are commonly found in cavities in volcanic rock, where silica-rich water deposited quartz crystals layer by layer parallel to the cavity wall. Agates are typically white, gray, and yellowish-brown to reddish-brown, but shades of blue and green are sometimes also observed. The myriad colors and patterns have given rise to many agate names, such as eye agate, lace agate, and moss agate. Its toughness and beauty make it a popular gem for jewelry and carvings. Although agates occur worldwide, Brazil is the major commercial source.



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