Identifying and Pricing HTTP-Treated Diamonds

Published March 2009

By Richard B. Drucker, GG
President of Gemworld International, Inc.

In a diversion away from colored gems, this article will delve into the most recent of diamond enhancement methods—HTHP. The last issue discussed clarity enhanced diamonds and this one will focus on a different enhancement. Colored gem dealers often encounter diamonds in the jewelry that their colored gems are mounted in, therefore, the need to recognize all the issues in the industry.

Natural diamonds may be treated to improve their colorless appearance or to produce a variety of colors. The most common method used today is the High Temperature, High Pressure method, or HTHP. Other methods include irradiation, annealing, and combinations of treatment methods. Because of this, the identification of treated diamonds poses the greatest challenge to the gem labs today. Since HTHP is the most commonly encountered, we will focus on that treatment here.

The treatment is very effective. It can improve the color from a low color (below M) tinted brown, to a very high D-E-F color range. The off-color brown diamonds that are technically classified as TYPE IIa are candidates for color improvement. Only about 2% of all mined diamonds are this type classification. But, when successful, these diamonds are improved to the highest of color grades. However, certain diamonds may also be treated by HTHP to other fancy colors such as blue and even pink, though these are more difficult to treat to these colors.

Identification of these treatments can be very difficult to the average jeweler or gemologist. Sometimes, telltale signs of the treatment will be found because of the high heat and pressure. The photo shows a stress halo around an inclusion. Unfortunately, not all diamonds will show features such as this. Companies that follow full disclosure will laser inscribe the girdle. Laboratories have more sophisticated testing methods available and most HTHP diamonds that are submitted to laboratories are accurately identified as such.

Pricing of HTHP colorless diamonds is thought to be about 30-50% below the price of an untreated diamond. However, few diamonds are found in the open market, so exact pricing information is difficult to obtain. When you consider that these diamonds began as off color, below M, the transformation to D, E or F color suggests that the discount should be at least this much if not more. One company sells these diamonds with a brand trademark and these diamonds are generally sold at retail for only a slight discount. Also, note that there are not set discount guidelines to follow because of differences in availability and demand. A diamond under one carat might be discounted more than one that is five carats for example.

HTHP colored diamonds are more common and several companies currently market these. One company markets HTHP treated red diamonds. Currently, their prices are about the same as a G-VS2 so this can be used as a guideline for pricing. Yellow diamonds are clearly the most available of all HTHP treated diamonds. The yellows sell for about 40%-65% below the price of its natural color counterpart.

Richard Drucker is President of Gemworld International, Inc. Gemworld International is the producer of the GemGuide, a wholesale diamond and colored gem pricing publication since 1982. The GemGuide is distributed to more than 25 countries and is a standard in gem pricing, quoted in every major trade publication. For information on the GemGuide to wholesale gem pricing, or the GemGuide Appraisal Software, go to www.gemguide.com.

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