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Smithsonian National Museum of Natural History

Bulls Eye Agate

Photograph of bullseye agate (46832) from the National Mineral Collection
Photo by Chip Clark. Click to zoom.
Catalog Number NMNH 46832-00
Locality Brazil

Chalcedony is the general term used for all varieties of fine-grained quartz that are built up of thin layers of fibrous quartz crystals. It is typically translucent and has a waxy texture. Agate, with its often dramatic concentric color bands, is probably the most familiar type of chalcedony. Its toughness and beauty make it a popular gemstone among lapidaries, and it is often used as an ornamental stone for carvings, beads, and cabochons. Agates are typically white, gray, and yellowish-brown to reddish-brown, but shades of blue and green are sometimes also observed. The colors are mostly caused by tiny crystals of iron and manganese oxide minerals. Agates are commonly found in cavities in volcanic rock, where silica-rich water deposited quartz crystals layer by layer parallel to the cavity wall.


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