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Agate

a type of quartz known as striated chalcedony

Agate consists of silicon and oxygen, giving it the chemical formula SiO2. It has a specific gravity of 2.58-2.64, and a hardness of 7-9 on the Mohs Scale. The mineral was named by Greek philosopher Theophrastus, who found agates along the river Achates sometime between the 4th and 3rd centuries B.C.

Agate forms when silica in solution seeps into a cavity in lava and hardens in successive layers. Agate is harder than lava, so as the lava wears away the agates are left behind, usually as rounded pebbles. Agates are usually found in stream beds and gravel banks where volcanic activity was once common, especially in Brazil, Egypt, Germany, India, Italy, Mexico, Nepal, and the Lake Superior region of the United States.

Agate was once believed to have magical powers, capable of making someone tell the truth and of improving one's memory and concentration. It is the mythical birthstone for September, the birthstone for the Zodiac sign Gemini, and the traditional gemstone gift for 12th and 14th wedding anniversaries.

banded agate
banded agate

Montana moss agate
Montana moss agate

polished agates
polished agates

PRINT SOURCE
E.L. Jordan. Hammond Nature Atlas of North America. New York: Hammond Incorporated, 1968.

WEB SOURCES
Bernardine Fine Art Jewelry www.bernardine.com/gemstones/agates.htm
International Colored Gemstone Association www.gemstone.org/gem-by-gem/english/agate.html

SEE ALSO
Mohs Hardness Scale

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The Robinson Library >> Science >> Geology >> Mineralogy

This page was last updated on 07/14/2017.