The art of coloring chalcedony started back in old Roman times

Q: Since you know a lot about gemstones, I hope you can answer something that has me confused. I love Black Onyx, but when I select it on your, and other web sites, it shows that it is dyed. 

Is Black Onyx real or a dyed rock? Also, when I visited Texas last year, I bought a souvenir, Mexican donkey, made of “ONYX”, but this stone is a soft, banded, tan stone that looks nothing like my Black Onyx. Is this Onyx, too? As I said, I am confused.

Thanks for your newsletter. I hope you will answer my question. J.D.

A: I don’t want to confuse you, but BLACK ONYX is real, it is dyed, and your donkey is also ONYX. Let me explain.

BLACK ONYX is natural, gray chalcedony with an added black pigment. Believe it, or not, chalcedony is slightly porous and can absorb liquids. In the case of BLACK ONYX, gray chalcedony is placed in a vat of concentrated sugar or honey solution and the solution is absorbed into the stone. 

Then the sugar saturated, chalcedony is placed in heated sulfuric acid. If you remember high school chemistry, the acid reduces the sugar, in the chalcedony, to pure, black, carbon, producing BLACK ONYX. Pure, black carbon is one nature’s most stable pigments.

The art of coloring chalcedony started back in old Roman times and was perfected in Idar-Oberstein, Germany, almost 200 years ago. Idar-Oberstein is one of the most famous, and oldest, agate (chalcedony) and gem cutting centers in Europe. 

The Germans developed numerous processes to produce various pigmented colors in chalcedony. By using inorganic pigments they produced permanent colors. Where as, when organic colors and dyes are used, they tend to fade in sunlight.

Although the German processes are secret, here are a few chemicals that can produce pigmented colors in chalcedony:

BLUE ONYX (or agate), Iron is the pigment. A solution of yellow potassium Ferro-cyanide id absorbed, then the saturated chalcedony is boiled in hydrous iron sulphate.

GREEN ONYX (agate), Iron is the pigment. A chromium salt solution is
absorbed, then the saturated chalcedony is given a heat treatment.

RED ONYX (or agate), Iron oxide is the pigment. An iron nitrate solution is absorbed, then the saturated chalcedony is “fired” (heated)

CIBJO, an organization that sets standards in the jewelry industry, has stated, “chalcedony and banded agates which have received a permanent and irreversible color change through heat treatment or with an acid or corrosive” DO NOT have to have a label stating “treated” when sold or used in commercial purposes.

Your souvenir ONYX donkey is crafted from ONYX MARBLE. Unlike BLACK ONYX, which is achalcedony (Quartz family), ONYX MARBLE is a banded limestone formed with either calcite or aragonite. ONYX MARBLE should not be confused with chalcedony, and should never be called just ONYX.


http://blogs.houseofgems.com/index.php/2009/12/black-onyx-real-or-fake/