There are a multitude of gemstone treatments on the market, which allows for much less expensive jewelry purchases. The downside to this is that often treatments are not disclosed and the buyer pays far too much for an inferior gemstone, often without the knowledge of the jeweler who is selling it. The following is a summary of gemstone treatments commonly practiced today, and a little on how these treatments affect the value of the finished gem.
Gemstones have been heated for thousands of years when it was found that heating of some gem material can cause a deepening of color, a lightening of color, and enhancement of clarity. There are many gemstones that are commonly found in the ground that underwent a significant period of heating after their initial formation. In this type of gem, it cannot be ascertained with certainty whether the stone was heated naturally or by man. In this case the value of the cut gem is not affected by heating. The following table lists some common gemstones and the impact that heating has on their value.
|Gemstone Variety||Effect of heating||Value compared to unheated of equivalent color and clarity|
|Amethyst||Changes purple to yellow||Loses half its value|
|Aquamarine||Deepens color||No impact|
|Garnet||No effect||No impact|
|Oregon Sunstone||No effect||No impact|
|Ruby||Clears inclusions, can change color||Loses about half its value.|
|Sapphire||Changes color, clears inclusions||Loses about half its value|
|Spinel||Clears inclusions||Loses about half its value|
|Tanzanite||Changes color||No impact|
|Tourmaline||Lightens color||No impact|
|Zircon||Lightens color||No impact|
Usually, to determine whether a gemstone has been heated or not requires a microscope and knowledge of what to look for. This is especially important in the case of ruby, sapphire, and spinel as these are expensive stones and lose a lot of value once heated. This is why we send our expensive rubies and sapphires to GIA to determine whether the stone was heated or otherwise treated in the rough. Most gems are not heated in the rough, but rather only after they are cut. Tanzanite is an excellent example of this. In fact, tanzanite is one case where if the color is not desirable, we may send it to be heated. In such cases we always disclose the heat treatment. Aquamarine can become a much deeper blue if heated and the value increases significantly.
There are a few gemstones on the market that owe their color to irradiation. Prasiolite is a green quartz that has been irradiated, as this color does not occur in nature. Swiss Blue Topaz, and London Blue Topaz are also irradiated stones. It is possible to find London Blue Topaz that came out of the ground with the dark natural color, but these stones are exceedingly rare. Irradiation is normally performed on colorless gem material to give it the pleasing colors that we see on the market. Irradiated gems are not valuable and RSA Gems does not purchase such gem rough for cutting. There is also some danger associated with irradiated gems. Once treated with radiation, a gemstone is supposed to be safely stored for a minimum of two years prior to selling it. There have been reports of radiation burns resulting from wearing jewelry with a recently irradiated gemstone.
Most emeralds on the market have been oiled, which is a form of fracture filling. The stone is placed under high pressure, and a clear oil that has the same refractive index as the stone is introduced. The oil flows into any cracks that break the surface, repairing (actually hiding is a better term) the cracks, sometimes deeply into the stone. It can be tricky to re-cut emeralds because the oil is likely to leak out during cutting, leaving a terribly included stone that will require re-oiling in order to sell it. Oiling is completely accepted in the jewelry market and a stone that has not been oiled is rare and worth much more than its oiled counterparts.
Rubies and Sapphires may be fracture filled with leaded glass during heat treatments. The glass is the same density as the ruby or sapphire so it cannot be detected by weighing the stone. It is also colorless so is not easily detectable without a microscope. This is probably the most commonly committed fraud in the gem and jewelry industry worldwide. Many unscrupulous mine owners are known to send very poor quality rough stones to China or Thailand for heating and fracture filling. Then the stones are sent back to the mine where they are added to the rough material coming out of the mine. Thus, unwary or inexperienced rough dealers or tourist collectors are sold this material as if it was just coming out of the mine in that condition. Fracture filled ruby may be worth as little as one percent the value of natural ruby.
A composite stone is one that was heated and assembled in layers held together by leaded glass. This is a manufactured product consisting of natural stone, usually of unsellable quality and may contain as much as half its weight in glass! This is a very inexpensive product.
This is practiced mainly in China, where dye is forced into the stone on the molecular level to permeate an otherwise colorless gem into one with beautiful, but unnatural color. A dye diffused stone is worth very little and should be relegated to costume jewelry.
While not a treatment on natural stone, these gems are probably the most commonly misunderstood by the public at large. Go to any chain jewelry store and you will see large jewelry pieces with many small rubies, sapphires, emeralds and the like. You can tell by the price tag that these are manmade gems. The sales people in these stores will say you are purchasing a ruby, sapphire, or emerald as the case may be and they will rarely tell you the stones are worth less than 1/1000th of their natural counterparts.
So what is the gemstone buyer to do to be sure the expensive gem they are purchasing is authentic, and untreated? The best thing you can do is to request a GIA report for the stone. If the seller is unwilling to get one (really, for an expensive gem the seller should already have obtained one) then you should pass on the purchase. Don’t take anyone’s word for it when it comes to big rubies and sapphires in particular. While we don’t have GIA reports for our less expensive stones we will be happy to send any stone to GIA on your behalf (and your expense) that you purchase from us. The process usually takes 2 to 3 weeks for GIA to complete.