The Story of Hydrothermal Ametrine

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The Story of Hydrothermal Ametrine

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This one from the ISG will tell you everything you need to know to recognize lab-grown hydrothermal ametrine. :)

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The Story of Hydrothermal Ametrine

Unusual colors and diagnostic inclusions help in identification

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Over the past few years there has been a growing abundance of exotic ametrine gemstones become available on the market. For those of us who have been around the business for a while, these strange colors appear to be outside the normal color range of natural ametrine.

In fact, natural ametrine is quite rare and has been for quite some time. Of course, it is always possible that a new mine discovery could have been made that produces these exotic colors, but as we know this would produce available rough stones that could be evaluated and verified for natural authenticity. In the case of natural ametrine, named for the combination of amethyst and citrine colors within a single quartz crystal, there is a fairly large availability of rough stones that attest to the authenticity of mine locations in Bolivia, Brazil and others.

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In the case of these strange and exotic ametrines, however, natural rough is unavailable which, as we have learned with other gemstone investigations, is a strong indicator that something is amiss with the claims of natural origin. As seen at left, this "ametrine" offers a green color that is generally associated with radiation treatment.

The color of this stone is well outside of the normally seen natural colors for ametrine, and the price of this stone was very cheap in comparison to market values of natural ametrine from South America. And price can be a strong indicator of additional research needed to the savvy gem buyer.

Hydrothermal Lab Created Quartz

As seen below, hydrothermally grown quartz has been on the market for decades. The original production of this material was based on war time needs for perfect quartz crystals to operate electronics. Since natural crystals are rarely perfect in formation, the electronics manufacturers turned to hydrothermal synthesis to create technically perfect quartz crystals for the electronics needs. The hydrothermal process uses a hot water environment rich with formative elements that crystallize on a seed plate to form the complete quartz crystal. Over time, it was realized that this process could be used to create quartz based gemstones such as amethyst as seen below. With the addition of citrine colors and learning how to mix the two, it was possible to artificially create ametrine colors.

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Using this hydrothermal process the industry has been able to create other gemstones such as emerald, aquamarine, bixbite, sapphire and others. The one main difference is that hydrothermally grown gemstones other than the quartz stones are fairly easy to identify based on a growth structure as seen at left with this hydrothermal emerald.

The pattern you see in this image can also be seen in a variety of intensities in the other gemstones, making the identification of hydrothermally grown gems of these categories an easy identification. However, since the quartz production has been going on so long and been so well perfected, this growth structure has been removed from that process leaving the gemologist without that important feature for identification. But in some cases, there are others....

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Some of the most creative sellers of this material are actually providing full disclosure and selling it as unique lab produced gemstones with color features that nature cannot duplicate. The ametrine seen at left was sold with full disclosure of the origin of the stone, with the additional sales feature that this specimen even includes the seed plate creating a color pattern that is quite unique.

This type of creativity is exactly what the market needs as these are wonderful lab created gemstones with a great future for the market. But great care must be taken to ensure proper disclosure so damage is not done to the natural ametrine production industry.

As you look at close up images of the seed plate in the images below you can see what appears to be a lot of dust on the stone. This is actually a large cloud of "bread crumb" inclusions in this stone that we have found in many ametrines of this color. These "bread crumb" inclusions have been documented as being very useful for identification of hydrothermal grown quartz by Koivula and Gubelin in the Photoatlas of Inclusions in Gemstones Vol. 2 page 660.

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At left you see a 10x view of these bread crumb inclusions that in this specimen are throughout the stone. Care should be taken on this issue, however, as there is no guarantee to find bread crumb type inclusions in all hydrothermal quartz. So if you find them you have a very strong indicator. If you don't find them, that means nothing. Be careful about this issue because not all hydrothermal quartz such as amethyst or citrine will have these inclusions. High magnification image using our Meiji Techno microscope in dark field light is seen below.

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Another indicator that we have found (and we stress the word "indicator") is the strength and colors of the bull's eye interference figure seen in these hydrothermal ametrines. This usually requires one to be well experienced with a polariscope to understand. With the hydrothermal quartz in general we have found a tendency toward extremely strong bull's eye figures with very bright and sometimes unusual rainbow colors. Difficult to photograph due to the slight optical variables, but for the trained gemologist who is experienced with quartz, the bull's eye figure of the hydrothermal quartz is a very important indicator. Below you see two figures of one of our specimens showing the intense red and green colors below left, and the bull's eye interference figure below right. We want to stress that all specimens in this report are in the possession of the ISG office.

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The importance of lab created gemstones to the coming generations of this industry is without question. But in order to protect the natural gemstone production industry it is critical that lab created gemstones receive proper disclosure at all levels of this industry.

We welcome the advancements of the lab created gemstone industry because without a long term supply of product, we will not be able to sustain a viable gemstone market. And with the growing world demand for gemstones, we need lab produced gemstones to be able to grow.

In the case of these unusual and exotic ametrine, the identification is fairly straight forward. The price is cheap, there is no natural rough on the market, and the colors are not like any known commercially produced natural.

The important part is that they are pretty, affordable, and create great revenue streams for the jewelry industry if properly represented to our consumers.

Lets' work to maintain that direction in the market. Its the only way we can protect this industry for future generations of gem dealers


Robert James FGA, GG
President, International School of Gemology
Jewelry Adjuster Insurance Services
Property and Casualty Adjuster, Texas Department of Insurance #1300433
Certified Continuing Education Provider, Texas Department of Insurance
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