Color and Clarity Techniques Used in Colored Gemstones

In my last post we talked about Natural and Artificial Gemstones and how fake gemstones are represented as real gemstones. Talking about fraudulent practices in the industry, we come to the ones relating to color and clarity enhancement. What you need to understand first is that such techniques are used in the colored stone industry every day, but they do not constitute fraud. As I have mentioned before, some techniques have a permanent effect, where as some wear down with time. These are the ones that have been termed in the trade as unacceptable, for the simple reason that they facilitate the owner to pass of the gem as that of a better quality. Here I will discuss all the practices that may be considered deceptive or unacceptable:

Heating

Like I said, heating is not wrong if applied on the following gemstones:

Other than these, if there is evidence that heating has been used on a stone, the effect will not be permanent.

Diffusion

This process has been used on rubies and sapphires and the treated stones have been mixed indiscriminately with the original specimens. So be especially vigilant of diffusion-treated rubies and emeralds. Topazes, on the other hand, do not have such a murky past. However, a diffusion-treated Topaz is around one-tenth the value of its natural counterpart that has just been subjected to heating. So that calls for special care too!

Radiation

This method gives stable results for the following stones-

Some blue topazes have even shown harmful radioactive effects, but since 1992 strict regulations have been established for allowable radiations levels. So the problem does not persist any more. Anyway, if you find that any stone other than the above mentioned ones has been subjected to radiation, rest assured that the affect will be temporary. Avoid such stones.

Dyeing

Jades, corals and to some extent even substandard quality rubies, sapphires and emeralds are dyed. Dyeing might be very stable, but in some cases, it comes off with a simple application of cotton moistened in fingernail polish remover! Here are stone that are frequently dyed-

Here are the stone that are frequently faked using dyeing techniques, generally on chalcedonies-

Blackening

Chemical reactions are introduced into colored stones that create black carbon molecules in order to darken the shade of the color. The following stones are faked in terms of intensity using this shade-

  • Black opal: blackening white opal
  • Black onyx: blackened chalcedony

Foil-backing

This is a technique which exploits the advantage of a closed-back mounting. The faceted or unfaceted stone is set into a closed-back set, such as a bezel set with an inner lining of gold or silver (for a rich and sparkling display) or some other colored foil. So, even if the stone is practically colorless, you will see a brilliant display of color. This method is not being used these days (better methods have evolved), but it is fairly common in antique jewelry. So when you see an antique piece of closed-back emerald, make sure to get the thing opened – the thing might not turn out to be even an emerald. The reason I insist on getting it unmounted is that even an experienced gemologist would not be able to detect such a malpractice, if properly done, with the stone embedded.

Smoking

This process is used normally on opals only. The stone is cut and polished and after being tightly wrapped up in brown paper, is placed in a closed container with moderate heat until the paper is completely charred. Then the stone is cooled and removed. After this process has been repeated sufficient number of times, a smoke-produced brown color coating is obtained on the otherwise pale opal. This fire of the opal decreases when wet and comes back to normal when it is dry again. This simple test should tell you if the opal has been smoked.

Painting

This method is generally used with unfaceted (cabochon) stones or translucent opals. The stone is basically mounted on a bezel set after coating the inside with a black paint or cement. So what happens is that the light entering through the stone cannot leave sideways, and gets reflected to give the effect of a deeply colored black opal. There is only one way of having a genuine black opal, but at least a thousand ways of not having it but thinking that you do!

Waxing and Oiling

Waxing involves rubbing a tinted waxy substance on the surface of the stone so that its cracks are not apparent. Oiling involves soaking the stone in tinted of colorless oil so that the cracks are filled up in a similar way. Waxing is done in case of star rubies and star sapphires and oiling is generally for emeralds. Waxing is an altogether unacceptable process. Oiling, on the other hand, is generally accepted, but only if the oil is not tinted. The oil would happily do its job for years together unless the stone is immersed in another organic solvent such as gasoline, in which case the stone can be cleansed and oiled again.

Fracture-filling

Filling the surface cracks may also be accomplished using liquid glass or some kind of epoxy-resin filler. These fillers might or might not be tinted with coloring agents. In such a case, the filler not only maintains surface continuity but also enhances color quality. Selling such a gem without disclosure of the treatment is a malpractice. Nonetheless, circulation of glass-filled rubies and resin-filled emeralds is increasing by the day.

So these were some of the most common color enhancement techniques employed in the colored gem trade. There is not much you can do on your own except being well-informed, which you are now!

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Category: Color and Clarity

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