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Formation
Agates are produced from silica, but the exact circumstances of their formation are, in 2011, open to much scientific debate. Scientists do know that agates begin life inside cavities in rocks such as the volcanic rock basalt. When these rocks erupt and come to the surface, they're filled with water and carbon dioxide. Eventually, the silica inside begins to filter out through the holes in the rock and crystallizes, causing an agate to fill the cavity.
Physical Appearance
Because of the way in which they form inside rock cavities, agates are often round in shape to match the hole they originated within. Agates are also recognizable by the bands across their surface, which resemble tree trunk rings; these bands can appear to humans as shapes such as eyes. Agates come in a range of colors, from amber and blue to green and purple. Some are found with what are termed drusy, small bits of quartz crystal, within the agate; these are sometimes used separately by jewelers.
Distribution
Agates are found all over the Earth. Within North America, states such as Wyoming, Montana and Tennessee are known for the agates that are commonly found there. These gemstones are often found in Mexico, too.
Uses
Agates are often used in jewelry, for example as part of rings and necklaces. The cost of such jewelry varies depending on the variety of agates used. Though in general agates aren't especially expensive, as noted by the Bernadine Fine Art Jewelry website, agates with distinctive or well-defined patterns can cost more, and specific types such as plume agates are relatively pricey to buy. Agates also appear in furniture and household ornaments.
Relevance to Past Civilizations
Agates have assumed a place in the cultures of several civilizations since ancient times. Items fitted with agate gemstones were viewed as talismans in the Persian culture, where the agate was associated with preventing storms. Objects containing agates were popular with European royalty in the Renaissance, as well as with rulers in the Byzantine Empire.
In Folklore
Traditionally, agates have been associated in folklore with healing and truth, as well as calming emotions and protection. Within the zodiac, the agate is the birth stone for those born under the sign of Gemini. Agates are sometimes given as gifts for 12th or 14th wedding anniversary celebrations.