HPHT | Which diamonds can be subjected to it and what are its effects?
HPHT, or High Pressure High Temperature, is an attempt to mimic the natural environment which produces diamonds 150km deep in the earth’s crust, under extreme heat and pressure – some 2000 degrees Celsius and 58,000 atmospheres – with aim of changing the crystal formation of the diamond, or create a synthetic diamond from graphite.
This article will explore the effect of HPHT on the color of a natural diamond.
The tremendous pressure applied upon a diamond during the HPHT process equals applying a 70-ton pressure on one square centimeter (see diagram 1). To illustrate the intensity of the heat – iron melts at 1,538 degrees Celsius and carbon’s melting points is 3,500 degrees Celsius. Such radical procedures change the molecular structure of carbon, which can therefore re-crystallize into a diamond.
A diamond’s growth period in nature is a bona fide mystery. Attempts at carbon dating diamonds based on 14 carbon inclusions have failed; but tests performed on diamond deposits indicate that the vast majority of natural diamonds are 1-3 billion years old – 25-75% of the earth’s age.
The attempt to imitate a billions-of-years-old process in a laboratory is anything but simple.
Color revealing
Before discussing the effects of HPHT on diamonds’ color, let’s review the frequency of various diamond types in nature: the majority of natural diamonds are type Ia diamonds, and are of a pale yellow or brown color. Allegedly, if we know a diamond’s initial color, we can predict the final color produced by the HPHT process. However, various combinations may also produce the same result. For example, brown can be created by mixing yellow (carbon centers with distinct/ isolated nitrogen atoms) or N3 centers (three linked nitrogen atoms) and black, which stems from microscopic graphite inclusions. Brown can also be created by mixing nitrogen centers which form green and red colors.
This leads to the conclusion that the end-color derived from the HPHT process is hard to predict, and a thorough spectrophotometric analysis to determine the exact nature of the stone’s centers is needed.
Type IIa gems are the best candidates for yielding colorless stones. Relatively uncommon in nature (see chart 1), type IIa diamonds are characterized by nitrogen impurities less than ~5 ppm (particles per million). Their brown hue is attributed to plastic deformation, sliding, yawing or shearing between the diamond crystalloids.
These deformities, which render a diamond’s crystalline structure incomplete, can result in orange, pink, yellow and brown hues in the stone. The HPHT process can change the crystalline structure by increasing the movement of atoms and centers, thus improving crystalline integrity.
We know that the higher a diamond’s crystalline integrity is, the more colorless it becomes. The HPHT process, which involves high temperatures and pressure, also causes molecular changes in the alignment of the diamond’s centers, by increasing the mobility of nitrogen atoms in fraction scopes of stable impurity centers A and B, and turning them by dispersing atom concentrations to other centers, which change the color effect (see chart 2).
In order to derive the brightest, most vibrant colors, we must increase the temperature and pressure to the maximum, almost to the point of turning the diamond into graphite.
Technological advances allow us to arrive at the same results under lower temperature and pressure conditions, by using pure metallic catalysts, tantalum, iridium, a nickel-iron alloy, etc.
Chart 3 gives a schematic review of the limits and “work field” which allows a diamond to “survive” the HPHT process. Illustration 2 demonstrates the results of GIA color sampling on 858 type IIa gems which were subjected to HPHT. Result analysis indicates that 80% of the stones were “colorless” or “near-colorless,” in a D-G grading, after being subjected to HPHT. Some 28% graduated the process with slight grey or brown hues.
Risks and Conclusions
1. The HPHT process can be applied only to polished diamonds. The processed diamond must be laser marked. Erasing the laser mark is a violation of the World Federation of Diamond Bourses code and is considered fraud;
2. Diamonds lose 2-5% of their weight to the HPHT process (the part that turned into graphite);
3. There is some control over the end-color (the process is based on experience and statistics);
4. The process can turn a brown diamond into a colorless one;
5. The process can enhance a diamond’s color from “Light” to “Intense Vivid”;
6. The process may cause the total fragmentation of a diamond;
7. The HPHT process leave traces on diamonds, which are detectable by advanced equipment used by gemological laboratories (catalysts, irregular spectral lines, etc.);
8. When a gemological lab finds that a diamond has undergone HPHT, it will note “HPHT Processed”.
This information is based on recorded results of scientific experiments held by the Gemological Centers R&D Department
Ben-Zion Menashe has studied exact sciences in the Tel Aviv and Jerusalem universities, majoring in physics, chemistry and electronics. After spending many years in high-tech developments, he now heads GCI’s R&D Department.
Yehuda Yakar, a gemology industry pioneer, is head of GCI (formally known as IGC) and a 30-year veteran of academia. A GIA (NY) and SSEF (Switzerland) graduate, he is a world renowned expert in diamond grading, spotting imitations, diamond and gem enhancement and fancy color diamond grading.
IDI Magazine 25-7-2010
This article appeared in the HaYahalom magazine issue 197 in Hebrew.
By: Ben-Zion Menashe, Physics, DG GCI, & Yehuda Yakar, GG GIA
