|Yehuda Yakar, GG GIA, Chief Gemologist at GCI|
The coating, the coloring, the neutron or electron radiation, and the sequestering under high atmospheric pressures and temperatures (HPHT) – all of these are intended to change the color of natural diamonds, in order to increase their value and marketability. Some of these are reversible and illegal. Some of them are reversible and legal. All of them require disclosure.
High Pressure High Temperature is a process that serves to alter the color of a diamond. The process is composed of pressurized conditions and high temperatures (about 60,000 atmospheres and about 1,800 degrees Celsius). High pressure on its own, or high temperature on its own, could break the diamond apart or cause it to burn. The controlled combination of the two causes a color change.
The HPHT process can cause damage to the diamond because the conditions created by the process are very similar to the conditions that cause diamond to turn into graphite. The treatment is carried out on diamonds with a brown hue and of great clarity – VS and upwards, because the particles in the diamond become very vulnerable when they are exposed to heat and pressure. The stones that may undergo this process can be of widely varying weights, from 0.02 carats and up. In effect, stones of any size can undergo this process, and there is a record of a diamond weighing 50 carats that went through the process, although only a few machines in the world are suitable for treating diamonds over 10 carats. However, the issue is whether the process is financially worthwhile.
The pressure and heat conditions alter the order of the structural distortions in the diamond that create the brown appearance. At the beginning of the process it is impossible to know what color the stone will take on at the end of the process. The prediction depends upon probability. The existence of certain molecular connections in the stone portends the likelihood of a possible outcome, but sometimes a different outcome emerges. The existing knowledge in the world today regarding this topic is mainly based upon comparison studies and experiments by the machine operators who can, on the basis of statistics, predict with high accuracy the hue of the final result of the process. The process is carried out on a stone when it is partially polished. When the treatment is finished, the facets of the stone must be polished. The results of the treatment are permanent and irreversible.
HPHT treatments are characterized by certain side effects:
In a colorless diamond, one may find graphitization of particles due to exposure to heat, leftover burnt facets and in many cases, a 'charge' in the stone. Sometimes the side effects are only noticeable with fluoride lenses.
Identifying HPHT treatments is carried out by accredited gemologists with vast experience. Identification is a complex process, mainly based on experience and requires knowledge of gemology and physics and advanced machinery.
At times, the gemologist can see with the aid of a microscope burnt facets – sections that were not removed by polishing – or particles that became graphitized. First of all, the gemologist checks the type of diamond: does it belong to type IIa or type Ia (depending on the color of the stone). The machines geared to this task are the spectrophotometer and FTIR. The work of the spectrophotometer and the FTIR is similar: decoding the type of atomic structure inside the diamond is carried out by passing light rays at specifically measured energy levels and wavelengths though the diamond, and measuring them with sensors. The result is an absorption graph. The difference between these two instruments are that the spectrophotometer operates in the visible light spectrum UV-VIS-NIR (ultra-violent / visible light / and next to infra-red, between 100 nanometer to 1100 nanometer), and the FTIR operates in the invisible infra-red spectrum (between 1000 nanometer and 30,000 nanometer). If the diamond is Type IIa or Type Ia and of a Fancy Color, the analysis continues with advanced machinery such as photoluminescence, RAMAN and cathodoluminescence. These machines use laser rays which damage the stone. The ray causes the electrons to get over-excited and emit small amounts of energy that are absorbed by sensitive sensors and translated into an absorption graph. Optimal results are received when the diamond is analyzed after being frozen at minus-196 degrees Celsius in liquid nitrogen.
The photoluminescence machine absorbs light that results from radiation from an external source. The computer analyzes the contents and outputs a graph. The photoluminescence machine analyzes the dispersal of the radiation. These machines are extremely expensive, and are generally only found in gemological laboratories. Only a skilled person can analyze and explain the results of such tests.
Coordinated Diamond Treatment
The coordinated treatment of diamonds includes the HPHT process, accompanied by radiation by an electron accelerator, followed by annealing - heating at relatively lower temperatures of 600 degrees Celsius.
The result is diamonds of striking unique colors like red and hues of purple that cannot be achieved by any of the individual processes described above.
Respect it, but suspect it
As explained above, according to the laws of the international diamond industry, it is mandatory to disclose treatment at every stage of process, from the producer to the jewelry retailer via the diamond trader and the gemological laboratory. The laboratories are required to note this in their grading reports and certificates. The cash value of a treated diamond is always different from the value of a natural diamond.
In regards to a 'cheap' diamond with a yellowish or brown hue that has been upgraded using a treatment to a fancy color, the value of the diamond rises commensurately. But a diamond found in nature with the same fancy color will be much more valuable than the treated diamond, even ten times as valuable.
There are some companies that claim that the HPHT process is not even a treatment, but that it completes a process that already begin naturally. This position, and the fact that the process is difficult to identify, has made the process a divisive one. There are some laboratories who demand that the stone be laser-etched with the words: HPHT PROCESSED.
|Israel Diamond Institute||Enlarge|
|Israel Diamond Institute|
Remember, a certificate from a well-known and respected gemological laboratory is supposed to accompany every colored diamond that is marketed as natural. Also remember that one should be wary of any person that attempts to sell you a natural fancy diamond at a reduced price.
The treatments to change a diamond's color were developed and have evolved as a result of the increasing demand on the part of the general public for fancy colors, as natural colored diamonds are rare and much more expensive than colorless diamonds. That is why people developed methods that would satisfy the demands of the market. Due to their high prices and low availability, it is difficult to find diamonds of identical color, therefore their availability on the market due to the various treatments allows for new possibilities for jewelry designers and producers, as well as the general public.
As well, the treatments which create colored diamonds make them financially affordable, this allows more people to be the owners of colored diamonds. Buy them if you like them, but not as an investment or an expectation of future profits.
Warning: Due to changes that can occur to a treated diamond, it is recommended that one asks the jeweler at the time of purchase for instructions as to how to handle the piece of jewelry.
Remember: disclosure is mandatory at every stage of the marketing and trading process. If a diamond went through one kind of treatment or another to change its appearance or color, it must be disclosed as such in a clear and suitable manner in writing.