The

DIAMOND EDUCATION
CENTER

HPHT COLOR CHANGE

 

In March 1999, a rather laconic press release distributed jointly by two well-known companies set off a firestorm that the then-president of the Gemological Institute of America,  William E. Boyajian, described as the “the biggest gemological crisis” in his 23 years at the organization.

The two firms involved were Lazare Kaplan International, a renowned diamond supplier in the United States that also is one of only a handful of companies in the sector  to have its shares publicly listed on the stock exchange, and GE, the technology conglomerate that decades before had been among the pioneers of creating synthetic diamonds for industrial use. They revealed that they had developed a process that could permanently change color in diamonds, and more importantly improve the color grade of stones in the standard non-fancy color range.

While at the time they revealed little about the process being used, the photos they supplied with the press release were quite dramatic, with the “before” shots showing brownish stones with a relatively low color grade, and the “after” photos showing what apparently were the same diamonds, but practically colorless. The concern among gemologists was not about who was involved—on the contrary, the credentials of both companies were impeccable – but rather that others may come to release such goods onto the market, without disclosing that they had been treated.

A diamond grading report issued by the Gemological Institute of America (GIA), indicating that the color of the stone had been modified through an HPHT treatment.

Within a relatively short period of time it was revealed that the color treatment involved a High Pressure-High Temperature (HPHT) process, in which diamonds are placed into special pressure chambers, and then subject to temperatures of about 2,000 degrees Celsius and pressures of about  of 60,000 atmospheres.

It also transpired that not all diamonds are affected in the same way by the HPHT treatments. Those diamonds that are likely to show improvement in the standard color ranges are exclusively Type IIa, and make up only about 1 percent to 2 percent of all goods in the marketplace. Containing no nitrogen, such diamond often have defects in their crystal lattices, resulting in them having a brownish hue. But when subject to HPHT treatment, these defects are fully or partially repaired, eliminating some or all of the brownish tinge.

On the other hand, HPHT treatment will transform brownish Type IIb diamonds, which only make up about 0.1 percent of goods in the marketplace, and contain little nitrogen but significant amounts of boron, to a blue fancy color.

In the case of Type IaA diamonds, where the nitrogen atoms are present but in pairs, and certain Type 1aB diamonds, HPHT treatment will transform brown stones into fancy yellow/orange or yellow-green stones. It does so by dispersing the paired nitrogen into single nitrogen molecules.

A BARS Press, which is the device most often used to achieve the conditions of high pressure and temperature required for HPHT treatments. Originally designed in Russia and popularized after the collapse of the Soviet Union, it replaced many of the much larger systems that has been used earlier in Western countries.

Because fancy colored stones are considerably rarer and more immediately noticeable, it was the HPHT-treated goods in the standard color ranges that have caused the most concern. But, within a short while, reliable detection techniques were developed, and today are part of most standard testing done when a diamond is submitting to a gem lab for grading.

Detection is a done generally through a two-stage process. HPHT-treated  diamonds in the standard ranges are Type IIa, and such diamonds can be screened out using infrared and UV/VIS spectroscopy, of which there are a number of inexpensive devices available in the market, such as De Beers’ DiamondSure device, HRD’s D-Screen and SSEF’s DiamondSpotter.

The second stage, which is performed only on goods screened out during the first stage, as well as on fancy-color Type IaA and Type 1aB diamonds, could involve a number of testing methods, including  a microscopic study of anomalous double refraction, deep UV fluorescence examination, fluorescence microscope and spectrometer and photoluminescence studies at room temperature and in liquid nitrogen immersion with a sensitive Raman spectrometer.