Agates are found as geodes within volcanic rock or ancient lavas. They are common semiprecious silica, in the chalcedony family of quartz, according to the Encyclopedia Britannica. Often used in jewelry as an ornamental gem, in many cultures the agate is worn to protect individuals from the “evil eye.” The stones, with distinctive concentric patterns and colors, are often dyed to produce richer colors. Although agates occur globally, the modern process of dying agates began in Germany.
According to Gemology Online, agates have been mined in Germany since the 1400s. The gem cutters in the Idar-Oberstein region of the country perfected the art of polishing agate during that century, at first using sandstone wheels that turned on an axle attached to a waterwheel. When the German agate mines were depleted in the 19th century, the gem cutters turned to Brazil for their agate.
According to Cerca Gems, the Idar-Oberstein natives have been dyeing agate since 1820, although there is evidence ancient Romans also dyed the stones. The former found that using inorganic pigments was more effective, since they wouldn’t fade as much as organic dyes. Different layers of the agate have different levels of porosity, and take the color differently. The dyeing process was a trade secret for many years.
According to Gemology.com, dyeing agate fist involves boiling the stone in a strong bicarbonate solution, then soaking the stone in a chemical solution. Different chemicals produce difference colors. On the Marbles Galore website, an article about dyeing agate references some 1944 issues of The Desert Magazine, in which the Idar-Oberstein dyeing method is revealed in detail.
To turn an agate red or carnelian, the gems must soak in an iron-nitrate solution, according to Gemology Online. According to Marbles Galore, because agate is crypto-crystalline, iron compounds can be introduced by absorption. Those in Idar-Oberstein made their iron nitrate by soaking iron nails in nitric acid. Once the nails dissolved, the agate stones would soak in the iron nitrate for about a month. Then the stones need to be slowly heated and slowly cooled to prevent breakage.
A blue agate is created by soaking in red or yellow prussiate of potassium, followed by soaking in iron sulphate solution, according to Gemology Online. According to Marbles Galore, the blue color was created in Idar-Oberstein by soaking in potassium ferro-cyanide for three weeks. Then the agates are soaked in a solution of ferrous sulfate for 10 days and laid out to dry in the sun. If the result is not blue enough, the ferrous sulfate soaking process is repeated.
According to Gemology Online, green agate is with a chromic acid or nickel nitrate solution. Marbles Online reports that for a green color, agates should soak for two weeks to two months in a saturated solution of potassium bi-chromate.
References and ResourcesCerca Gems: Chalcedony/Agate
Agate; Encyclopædia Britannica Online Library Edition; April 29, 2010
Gemology Online: Agates
Marbles Galore: How Agates are Dyed