Posted by Sharif Khan on 20th Jun 2022
CVD vs. HPHT Lab Grown Diamonds
Chemical vapor deposition (CVD) and high pressure, high temperature (HPHT) are two techniques used to make lab-grown diamonds. While both procedures create synthetic diamonds, it is crucial to know the differences between HPHT and CVD diamonds and how they affect diamond quality.
But before we jump to the differences, it is essential to understand the CVD and HPHT processes and the diamonds that result from them.
Chemical Vapor Deposition (CVD)
General Electric developed the first lab-grown diamond in 1955. DeBeers purchased the technology, which then vanished for many years. While the GE/DeBeers diamond was created using the HPHT method, it paved the way for further technological advancements that led to the development of CVD diamonds. As a result, CVD diamonds had become a reality by the 1980s.
Even though lab-grown diamonds have been available for a long time, CVD is a relatively recent method. Before the introduction of this procedure, labs depended on the High-Pressure, High Temperature (HPHT) method. CVD offers a much more efficient way of growing diamonds, given the extreme conditions needed in the HPHT process—about 1500°C and approximately 1.5 million pounds of pressure per square inch.
The Process
The CVD process is set in motion by putting a thin seed diamond inside a sealed chamber and exposing it to high temperatures—usually up to 800°C—in chemical vapor deposition. The chamber is then filled with a carbon-rich gas combination, typically hydrogen and methane. Ionization breaks away the molecular bonds in the gasses, allowing pure carbon to adhere to the diamond seed. As the carbon accumulates, it creates atomic bonds with the seed diamond, forming a new, bigger diamond that looks just like natural diamonds. Learn more about this process in this article with visual illustrations at Brilliant Earth.
Carbon atoms connect to the seed diamond layer by layer via the CVD method. This results in a stunning, genuine diamond. However, the process is sluggish, with bigger surfaces moving at a pace of 0.1-10 microns per hour (smaller surfaces grow at slower rates). The time it takes to produce a 1ct CVD diamond varies, but it is commonly estimated to take around a month.
As close to mined diamonds as it gets
CVD diamonds may be labeled "synthetic," but you should not let yourself be fooled by the name. These diamonds are similar to mined diamonds in every aspect, from their internal atomic structure to how they dazzle on your finger. In addition, they have the same appearance as "natural" diamonds, such that even an experienced jeweler may be unable to tell the difference.
This perspective is not one that only diamond makers put forward, as is usually stated. For example, the Federal Trade Commission (FTC) determined in 2018 that synthetic and real diamonds are, for all intents and purposes, the same thing: 100 percent diamond!
Now that you know the mechanism involved in CVD diamonds, you may think that a lab-grown CVD diamond would automatically be flawless—after all, it is made under regulated circumstances, right? However, even though the average CVD diamond is likely to be of a greater grade than the typical mined diamond, CVDs still have a wide range of quality. It is so because the mechanisms that make diamonds in nature are virtually the same in the lab. Also, as with natural diamonds, a certain level of chance is always involved.
As a result, after CVD diamonds complete their growth, they undergo the same certification process as mined diamonds. Qualified diamond certification labs measure and assess the diamonds' color, cut, clarity, and carat (the 4 Cs), and each diamond is given an overall grade.
High Pressure, High Temperature (HPHT)
The procedure of manufacturing high-pressure, high-temperature (HPHT) diamonds, invented in the 1950s to make the diamond business more profitable, is now detrimental to the industry. HPHT diamonds are less costly than natural diamonds and have a much better color after this treatment.
Companies buy less attractive diamonds for a lower price, put them through the HPHT process, and then sell the resultant (and better-looking) stones for much more. Manufacturers can use the HPHT technique to transform faulty or discolored diamonds into more attractive colorless, pink, blue, or canary yellow diamonds.
The process
HPHT diamonds are treated to extremely high temperatures and pressure within special machinery in a lab to mimic the process that occurs deep under the Earth's crust. Temperatures can reach 2,600 degrees Celsius to simulate the naturally existing heat in the soil required to form a genuine diamond. The high expense of the energy and machinery necessary to complete such a process culminates in a highly desirable and profitable product: a colorless diamond.
In HPHT growth, a carbon source, such as graphite or diamond powder, is combined with additional chemicals in the reactor chamber to aid diamond development atop a diamond seed. Using a molten metal catalyst—typically a combination of Fe, Ni, Co, or other elements—allows for lower-temperature development. This significantly minimizes the technological complexity of diamond growth and the costs incurred under HPHT conditions. HPHT grows at pressures of 5–6 GPa and temperatures of 1300–1600°C, approximately similar to the pressure imposed by a commercial jet airliner balanced on the tip of a person's finger.
HPHT growth works exactly like CVD diamond growth in that it creates a temperature differential between the carbon supply and the diamond growth seed. This allows the carbon atoms to spread through the molten flux and form a synthetic diamond crystal on the seed in the somewhat cooler region of the chamber. Color-causing impurities like nitrogen (yellow) or boron (blue) were common inside the development environment, which is why most early gem-quality HPHT synthetics were fancy colored. However, recent advancements in growth techniques have allowed for more precise control of impurity content, resulting in colorless crystals.
Issues with HPHT diamonds
These diamonds come with concerns of their own. Since their introduction to the diamond market, demand has surged as more people want to buy the "biggest and best" diamond. Thus, consumers may receive a bigger stone with a richer color for less than they would spend on a genuine diamond of the same specs with these improved stones, but at what cost?
Buying HPHT diamonds is not advised for a variety of reasons. Close observation of these improved stones reveals glimpses of color visible from the side of the stone. During the process, they also lose part of their natural weight and clarity. Worse yet, HPHT diamonds are magnetic, with some even being picked up by magnetic force. Moreover, given that the intensity of pressure and temperature required might cause diamonds with inclusions or fractures to burst during the procedure, HPHT can only be used on high-clarity diamonds—VVS1, VVS2, VS1, VS2, and flawless.
Many firms worldwide are experimenting with HPHT treatments, though not all are properly identifying these improved stones. As a result, distinguishing between HPHT and natural diamonds is becoming increasingly difficult for industry experts and GIA researchers. The GIA continues to develop new ways of identifying HPHT diamonds, but keeping up with the rapidly changing technology is difficult.
Differences between CVD and HPHT diamonds
Now that the steps involved in producing CVD and HPHT diamonds have been adequately explained, it is essential to understand the differences between them.
Morphology
The morphology of a lab-produced diamond is the most distinguishing feature between the two methods; in other words, how a diamond develops carries significance. The CVD diamond develops in a single direction, whereas the HPHT diamond grows in fourteen. Earth-mined diamonds may be distinguished from lab-created diamonds primarily by their growth patterns.
Inclusions
Black graphitic inclusions are common in CVD diamonds, while black flux inclusions are standard in HPHT diamonds. Although the inclusion material differs between the two methods, it makes no difference in diamond selection. In both circumstances, you are looking for a diamond that seems flawless to the naked eye. The idea that lab diamonds are more flawless than natural ones holds no ground and should be dispelled given the inclusions.
Quality
HPHT has long been linked with yellowish-colored diamonds; in many cases, it still is. This technology has been around for a while, and there is still a lot of outdated gear that produces low-quality goods. The CVD process was the first to make low-cost colorless diamonds. Both procedures may now produce flawless diamonds with no difference in the final result, except for the morphology.
Which of the two is better then, you may ask? Admittedly, you will not be able to identify the difference without using sophisticated optical tools. Therefore, the majority of these distinctions will be irrelevant to you. However, a lab-grown diamond is still a significant investment, so you should always seek a good certificate before purchasing. In this regard, we recommend purchasing a diamond with a certificate from any prestigious laboratory, notably the Gemological Institute of America (GIA) or the International Gemological Institute ( IGI).
Miscellaneous
- One of the most significant distinctions between HPHT and CVD is their manufacturing process or how they grow. HPHT diamonds, for example, develop in a cuboctahedron shape with 14 growth directions, whereas CVD diamonds grow cubic with only one growth direction.
- Diamonds created using the HPHT process are more yellowish and brownish in hue. They normally go through an extra HPHT procedure to eliminate the brown color: this additional treatment can only be applied to diamonds with a clarity of VS1 or higher. However, diamonds created using the CVD method are colorless.
- Overall, the HPHT approach is quite expensive since a significant amount of energy and sophisticated equipment are required. On the other hand, the CVD method is cheaper since it operates at a moderate temperature and low pressure.