The issue of heat-treatment is not just about disclosure. It’s also about value. A gemstone’s value depends on three factors: rarity, beauty, and durability. How much is based on rarity? If you think you know, you can probably answer this: “Should the price of a high-pressure/high-temperature-treated D, E, or F color diamond be the same or nearly the same as that of a natural colorless diamond?”

It takes less than two minutes for high pressure and high temperature (HPHT) to squeeze the brown color out of a diamond. But that brown diamond is an uncommon—even rare—Type IIa. It’s the geuda sapphire of the diamond industry because it takes only a little furnace heat to dramatically change the color. As discussed in Part II, heated geuda was priced the same or nearly the same as natural sapphire when it first hit the market. Now, it’s 30%-40% below natural-color Burmese.

Should the value of an HPHT-treated Type IIa diamond be based only on the original brown diamond’s value plus the cost of the technology and time needed to create a colorless or fancy color diamond? Or should pricing be based on the rarity of the starting material and, after treatment, its natural-color equivalent?

Or is an HPHT-treated diamond just another treated diamond? Should we lump all treated diamonds together?

Possible answers to those questions must take into consideration this critical factor: Not all HPHT-enhanced diamonds can be identified.

The Gemological Institute of America’s Gem Laboratory has reported on and documented nearly 11,000 HPHT-annealed diamonds. “The overwhelming majority of those have been declared [identified as treated] to us during the submission process,” says Tom Moses, GIA Gem Laboratory vice president, identification services. Although GIA does not know precisely how many undeclared HPHT diamonds have been submitted, it estimates the figure to be “a few percent” of the total.

“The majority of the undeclared HPHT diamonds have been colored—yellow to orangy yellow or in the greenish-yellow to yellow-green hue range,” says Moses. “Colorless to near colorless is the next largest population, and we have detected several pinks and a few blue treated diamonds.”

Moses says, “We believe we are still able to identify the vast majority of HPHT-annealed diamonds.” Still, “vast majority” doesn’t equal 100%, so a few treated diamonds could be getting a “natural” call or the kiss-of-death “color origin undeterminable” call. Adding to the problem is the fact that HPHT is no longer limited to Type IIa diamonds. According to GIA, the range of diamond types subjected to the annealing procedure continues to expand, thus making the end product more varied.

Non-Type IIa HPHT-treated diamonds are I, J, K, and L colors that are less valuable than colorless diamonds or pinks and blues. Thus, gemologists are less likely to tag them for further study. And even if they were tagged, they often are more difficult to detect, which increases the potential number of unidentified HPHT-enhanced diamonds. In addition, GIA says, “As sourcing for the ‘right’ starting material becomes more difficult, and more facilities are performing the service, the variety of the treated diamonds will continue to grow.”

The European Gem Lab USA Group also sees a limited number of HPHT-processed colorless/near colorless diamonds. “So far, only Type IIa and some Type IaB diamonds result in an improved color grade after HPHT modification,” notes Branko Deljanin, EGL’s director of Canadian operations. “Collectively, Type IIa and Type IaB diamonds represent less than 5% of the diamonds EGL USA certifies. Of these diamonds, approximately half are natural color, and the remaining are either HPHT-modified color or undetermined origin of color.”

This isn’t a concern just in the United States. “We are issuing Diamond Type Notes with all our Diamond Reports,” says Daniel Nyfeler, managing director of the Gübelin Gem Lab in Lucerne, Switzerland. “We feel that the type of a diamond is becoming more and more important to be identified and reported, especially now that many HPHT-treated diamonds are in the market. Our clients like this service, and we started recently to issue Diamond Type Notes also independently (i.e., without a Diamond Report)—however, only under certain conditions.”

Without overtly stating it, the GGL Diamond Type Report alerts potential buyers and sellers that a diamond, even though it’s been identified as having a natural color, may be HPHT enhanced but slipped by the lab staff undetected.

As the Gübelin report states, diamonds are classified into two fundamental groups based on the relative presence or absence of nitrogen incorporated into the crystal structure, as determined by the infrared spectrum. Type I diamonds contain appreciable concentrations of nitrogen, whereas Type II diamonds are chemically very pure and do not reveal infrared absorption characteristics related to nitrogen (although some have been shown via other scientific means to contain minute amounts of nitrogen).

A further separation of these two groups includes Type Ia (nitrogen atoms present in pairs or groups), Type Ib (isolated nitrogen atoms), Type IIa (no [easily] measurable traces of nitrogen), and Type IIb (traces of boron).

Based on the infrared spectrum, the report concludes by identifying the diamond’s type.

“Type Ia is not precise enough,” says Dr. Henry Hänni, director of the Swiss Gemmological Institute’s Laboratory SSEF in Basel. “Type Ia may show two forms of nitrogen aggregation—A and/or B aggregates. So finally we have IaA, IaB, IaAB and Ib. Most gem-quality diamonds are Type IaAB (so-called “Cape” diamonds). We have seen a pure Type IaB of D color in the lab. It turned out to be a natural color after the routine tests. If it would have been HPHT we would have detected it because we have reference data of untreated and HPHT-treated of this type.”

SSEF has been a leader in preliminary detection of HPHT-treated diamonds. The lab’s Type Spotter is a simple, small device that tests a diamond’s ability to transmit short-wave ultraviolet—the test that determines Type IIa, Type IIb, and Type IaB.

“The Spotter detects any diamond transparent under SWUV [short wave ultraviolet] light,” Hänni says. “They are Type II (including IIa and IIb) and pure Type IaB (with B aggregates only). All of these types are potentially HPHT treatable. After HPHT treatment they turn to colorless, pink, or blue. Therefore, the use of the Spotter is essential for any diamond dealer checking inventory for possible HPHT-treated diamonds.”

The spotter is useless for HPHT-treated Type IaAB diamonds, however. These stones turn to yellowish-green, greenish-yellow, yellow, orange, greenish-brown, or brownish-yellow when treated by HPHT. Type IaAB diamonds do not transmit SWUV, so the spotter does not identify them as a potentially treated stone.

“So far SSEF has sold about 500 Spotters,” notes staff gemologist Jean-Pierre Chalain. “When we created this tool, we only had one aim: protect the diamond market against the HPHT treatment. We were thinking that the Spotter would be sold to retailers and diamond wholesalers who would send all their stocked Type II colorless diamonds to a lab for an HPHT check.” SSEF discovered, however, that most of their spotters are purchased by major diamantaires sorting brown diamonds of Type II for further HPHT treatment. “Some of them, not the major ones, telephone us and ask if we can provide them with the address of a company that offers an HPHT facility,” says Chalain.

As Mother Nature intended? To make heat-treatment more palatable, treaters claim that it simply finishes what Mother Nature started. Dr. George Rossman, award-winning professor of mineralogy at the California Institute of Technology (Caltech) in Pasadena, Calif., takes the opposite view: “This is not finishing off what Mother Nature started.”

Consider sapphire formation. Sapphire created by metamorphic activity (colliding continents) involves temperatures of approximately 600°C to 800°C, says Rossman. Sapphire formed by basaltic action (heat from volcanic processes) involves temperatures of approximately 1,000°C to 1,100°C. “Conceivably, they could be formed at temperatures up to 1,400°C,” says Rossman.

But heat treatment takes place at 1,800°C, a huge difference from the temperatures of natural sapphire formation. Heat plays an extremely important role in the formation equation, Rossman explains. For every 10 degrees, there is significant change. “Going from 900°C to 1,000°C is incredibly significant, so going from 800°C to 1,800°C is enormous,” says Rossman. “Mother Nature could not do this, even given millions of years to do it.”

(There is a slight exception to the high-heat treatment rule for sapphires. Pink sapphires from Ilakaka, Madagascar, need only 400°C to 500°C heat to drive out the blue tint. There are no inclusions to melt or heal, so high heat isn’t used to treat these stones. Therefore, very little internal evidence of heat is produced.)

For diamond, comparing HPHT treatment, which uses pressure of 1 million pounds per square inch (psi), to the natural process is less problematic. “At 60 kbar pressure (found at a little over 200 km depth) the pressure is 870,000 psi, so 1 million psi is not unreasonable for a condition many diamonds actually experience in the earth,” Rossman says.

Some diamond formation may have taken place deeper than 200 km, but depths up to 200 km are more likely. Based on the Continental geotherm, that implies temperatures of less than 1,400°C.

That’s significantly less than the 1,800°C to 2,100°C under which HPHT treatment typically occurs. And while Rossman does acknowledge that there are diamonds that form at greater depths and higher temperatures, they are in the minority of diamonds studied. “We just don’t know for sure about diamond formation in the earth,” he says. “But it is clear that the vast majority of diamonds don’t reach this temperature.”

Not your father’s gemology. Over the past few months JCK has addressed heat-treated gems commonly found in the jewelry store. Heat treatment is performed mainly to enhance color but also can create greater transparency by dissolving inclusions or healing fissures.

Some enhanced gems can be identified as heated, while most others cannot. Some of the more important gems that are commonly heated may or may not be identified and disclosed as such.

The healing of fissures in diamond is not yet a concern for the gemologist. However, the common practice of healing fissures in corundum—and the potential of healed fissures in other colored gems—will likely keep gem laboratories busy for a decade.

Temperature high enough to heal fissures also can fuse stones together. Imagine a 10-ct. stone created by fusing two 5-ct. stones. Imagine the identification challenge that would present.

That raises other questions: When such healing takes place, how much of the stone has melted and recrystalized? At what point do such stones become partially or fully synthetic? How small would the crystals have to be before a gem is considered “reconstituted” as opposed to fused or healed, and should there be a difference? And if they can be identified, how will such gems be valued?

Part IV may be the end of JCK‘s heat treatment series, but it’s not the end of the heat-treatment problem. It’s only the beginning.