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Smithsonian National Museum of Natural History

Quartz (var. amethyst geode)

Photograph of amethyst star geode (149566) from the National Mineral Collection
Photo by Chip Clark. Click to zoom.
Catalog Number NMNH 149566-00
Locality Danialo Mine, Rio Grande Do Sul, Brazil

Quartz, one of the most abundant minerals in the Earth’s crust, is composed of silicon and oxygen and in its pure state is colorless. When iron replaces some of the silicon atoms, the color is purple. Purple quartz is called amethyst and is the most prized gem variety of quartz. Amethyst is found in a variety of geologic settings but most commonly as crystals lining cavities or voids in volcanic rock, forming geodes as seen here. The major sources for amethyst are Brazil and Uruguay.

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