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In nature, chalcedony can be a subtle variation of almost any color: red, orange, green, black and some forms of bluish-gray. Stronger colors, such as reddish orange, green and yellow may be natural or dyed.
The dyeing process reportedly was discovered in the 1800s in Idar-Oberstein, Germany, one of the world’s leading gem cutting centers.
The process involves the use of strong acids to “open the pores.” Then the chalcedony is submitted to a variety of repeated and prolonged immersions in metallic compound solutions that impart color. Reds, oranges and yellows are placed in iron oxide compounds; greens in chromium compounds; blues in cobalt compounds. Generally, dyed chalcedony has a vivid color and, under microscopic examination, can have concentrations of color along the edges.
Dyeing generally doesn’t affect the value of chalcedony, most of which is moderately priced. The treatment processes that impart color are considered permanent by the gemstone trade.
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