Amethyst
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These samples of amethyst are displayed in the Smithsonian Museum of Natural History. Amethyst is a variety of quartz with the characteristic composition SiO2, but distinctive and popular for its dramatic purple color. The sample at left is from Upper Providence, Pennsylvania and is about 10x8 cm. The gem is 36.2 carats.
"Quartz will commonly contain trace amounts of iron ( in the range of 10's to 100's parts per million of iron). The color in amethyst comes from color centers in the quartz. These are created when trace amounts of iron are irradiated ( from the natural radiation in the rocks)." (Minsocam)
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This dramatic amethyst-lined tube has front dimensions of about 35x30 cm and the displayed section is about a meter long!
" Some of this iron sits in sites normally occupied by silicon and some is interstitial ( in sites where there is normally not an atom). The iron is usually in the +3 valence state. Gamma ray radiation ( from nuclear decay in the surrounding rocks ) can knock an electron from an iron lattice site and deposit the electron in an interstitial iron. This +4 iron absorbs certain wavelengths (357 and 545 nanometers) of light causing the amethyst color. You need to have quartz that contains the right amounts of iron and then is subjected to enough natural radiation to cause the color centers to form." (Minsocam)
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The amethyst sample at left above is about 8x8 cm and is from Las Vigas, Vera Cruz, Mexico. The gems are 204.1 and 72.2 carats and are from Rio Grande do Sul, Brazil.
At left is amethyst formed on smoky quartz from Pondorf mine, Jefferson County, Montana. That sample is about 6x9 cm. The sample at right is about 10x20cm from Due West, South Carolina.
The amethyst gems at left are 41.2 and 61.4 carats and are from Brazil. The gems at right are 44.5 and 33.2 carats and are from Alexander County, North Carolina.
At left is an amethyst crystal formation about 4x5 cm from Virginia. The sample at right is about 5x17 cm and is from Cripple Creek, Colorado.
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This amehtyst sample shows the form known as scepter crystals. It is from Hopkinton, Rhode Island. The size is about 30 cm across.
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This amehtyst sample is from Rio Grande do Sul, Brazil. The size is about 35 cm across.
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