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Moss agate is asemi-preciousgemstone formed fromsilicon dioxide. It is a form ofchalcedony which includesminerals of a green color embedded in the stone, forming filaments and other patterns suggestive ofmoss.[1] The field is a clear or milky-whitequartz, and the included minerals are mainlyoxides ofmanganese oriron. It is not a true form ofagate, because it does not have concentric banding.[2]
Moss agate can be clear or milky white, with greendendritic inclusions that resemble moss. The colors are formed due to trace amounts of metal present as an impurity, such as chrome or iron. The metals can make different colors depending on their valence (oxidation state).[3] Despite its name, moss agate does not containorganic matter and is usually formed fromweatheredvolcanic rocks.[3]
Moss agate is found in countries across the world, including India, Brazil, Uruguay, central European countries, and the United States.[2] In the U.S., Montana moss agate is found in the alluvial gravels of theYellowstone River and its tributaries betweenSidney andBillings, Montana. It was originally formed in theYellowstone National Park area of Wyoming as a result of volcanic activity. In Montana moss agate the red color is the result of iron oxide and the black color is the result of manganese oxide.
The gemstone has also been known as "Mocha stone" after the Arabian city ofMocha in Yemen, once a source.[2]
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