How Are Agates Formed?
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Agates are semiprecious stones in the chalcedony category of minerals, a type of complicated quartz structure made from different fibrous bands of color that can range from white and gray to green, red, and blue, with a variety of translucent and opaque variations possible. Even though agates are approximately 90 percent quartz, they also include a carbon-like material called moganite, and together these minerals make the chalcedony core of the rock--although some layers of the agate are pure quartz, depending on how it was formed. The colors of the agate are caused by variations in impurities and crystal growth formation.
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Agates are created in igneous rock when it is subject to certain conditions. Igneous rock itself is formed by volcanic reaction, when molten rock is forced up through holes and cracks in the earth to form mountains, islands and other natural formations. This violent process creates spaces and bubbles in the rock as it cools, in the same way pumice and other volcanic minerals are formed. Some of these bubbles are solitary, while others are interconnected, and many are open to outside elements through cracks and holes in the rock.
When this kind of volcanic rock forms near water, the water is often forced into these pockets and bubbles. Since this water has been stirred up it is usually rife with other types of minerals, such as silica. When silica-rich water becomes trapped in these volcanic rock bubbles, it eventually forms an acidic type of gel, which begins to eat away at the surrounding igneous rock. The gel dissolves surrounding iron and other metals and leaves a layer of residue that settles into the gel. This layer is followed by other layers, until over time the gel loses water and the entire substance begins to crystallize.
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Eventually, the entire pocket of dissolved layers and crystallized gel hardens. If at some point water breaks open these bubbles and washes into them, the agate material will be broken apart and drawn into oceans, rivers and lakes. Over time this agate material becomes polished by the movement of the water, and washes up on the shores of oceans and rivers. Originally this was where the first agates were discovered and how their name was derived, being found alongside the river Achates, in Sicily. Today agates are mined in a variety of forms and locations to preserve their original beauty. Popular sites for commercial agate include Brazil, the Mediterranean, and the Northwest United States.
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