Professional Jeweler Archive: The Dyeing of Chalcedony

July 2000

For Your Staff/Selling Treated Gemstones


The Dyeing of Chalcedony

This form of quartz has caught the attention of jewelry afficionados in the past few years. To help you understand the gem and explain it to consumers, here's a primer


Chalcedony is the name given to all cryptocrystalline quartz, a complicated name that refers to the tightly packed microscopic crystals that make up the gem.

You may be more familiar with one type of chalcedony – agate – that contains curved or angular bands as seen in the photo below. But just as popular these days is a bluish/purplish gray chalcedony sometimes called mojave blue.

In nature, chalcedony can be a subtle variation of almost any color: red, orange, green, black and some forms of bluish-gray, including mojave blue. Stronger colors, such as reddish orange, green and yellow may be natural or dyed. Sky blue doesn’t exist in nature.

Though chalcedony’s crystals are tightly packed, it’s still a porous gem, which means it can be dyed easily. A detailed description of how chalcedony is dyed is offered in Gemstone Enhancement, a 1984 book by Dr. Kurt Nassau (Butterworth Publishers, Stoneham, MA). 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.

Black chalcedony – commonly (and incorrectly) referred to in the trade as “black onyx” – is dyed using a complex sugar and sulfuric acid treatment. The resulting carbonized sugar gives the chalcedony an opaque black appearance. (Onyx itself is a form of banded calcite and has a different chemical and crystallographic makeup than chalcedony.)

Introducing Enhancements

Because so much chalcedony is dyed, it’s easy to overlook disclosing the treatment to consumers. But don’t miss the chance to explain it because many customers will find the history and complex process of dyeing fascinating.

Your store may want to acquire a collection of natural and dyed chalcedony specimens so customers can see and understand the differences. Note that sometimes these differences are very hard to distinguish.

Your customer also may like to know dyeing generally doesn’t affect the value of chalcedony, most of which is moderately priced to begin with.

Special Care Warnings

Chalcedony is a relatively hard gem at 6.5-7 on the Mohs hardness scale. It has good toughness, even after treatment. The treatment processes that impart color are considered permanent.

Advice for Sales Associates

Learn and understand your store’s disclosure and return policies regarding dyed chalcedony. Your store may want to adopt a written policy regarding disclosure.


Recommended Reading

  • Gemstone Enhancement by Dr. Kurt Nassau, Butterworth Publishers, Stoneham, MA, 1984.
  • Gem Identification Made Easy by Antoinette Matlins, Gemstone Press, Woodstock, VT.
  • American Gem Trade Association Source Directory 1997/1998 (contains Gem Enhancement Manual), American Gem Trade Association, Dallas, TX.
  • American Gem Trade Association Gemstone Enhancements, What You Should Know, American Gem Trade Association, Dallas, TX.
  • The Guide, Reference Manual, Gemworld International Inc. Northbrook, IL.
  • Gemstone Buying Guide by Renée Newman. International Jewelry Publications, Los Angeles, CA.

Legal Considerations

The trade considers chalcedony enhancement to be permanent, but it’s still wise for you to disclose any treatment or enhancement that alters the original color of a gemstone. Remember that much chalcedony is dyed, so if you’re not certain, be prudent and err on the side of caution in advising consumers.

State laws allow consumers to sue if they feel you did not disclose properly or advise them about the gem and its proper care and protection. Letting consumers know what their beautiful gem has gone through before they buy it can avert legal action later. Disclosure need not be a painful process if you weave it into your sales presentation with candor and honesty.

Here is what the FTC Guidelines for the Jewelry Industry say:

Paragraph 23.22
Deception as to gemstones.
It is unfair or deceptive to fail to disclose that a gemstone has been treated in any manner that is not permanent or that creates special care requirements, and to fail to disclose that the treatment is not permanent, if such is the case. The following are examples of treatments that should be disclosed because they usually are not permanent or create special care requirements: coating, impregnation, irradiating, heating, use of nuclear bombardment, application of colored or colorless oil or epoxy-like resins, wax, plastic, or glass, surface diffusion or dyeing. This disclosure may be made at the point of sale, except that disclosure should be made in any solicitation where the product can be purchased without viewing (e.g., direct mail catalogs, on-line services) and in the case of televised shopping programs, on the air. If special care requirements for a gemstone arise because the gemstone has been treated, it is recommended that the seller disclose the special care requirements to the purchaser.

– by Robert Weldon, G.G.

This large section of agate was originally a banded white material. The piece was then sectioned and individually dyed different colors. Most of these strong colors don’t exist in natural colored agate.

Copyright © 2001 by Bond Communications