CVD Vs HPHT – Lab Grown Diamonds

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By

Bryan Boyne (g.g.)

Laboratory grown diamonds, Lab created diamonds, Lab diamonds, LGD, – all terms for the synthetic diamonds that have arrived on the market in large quantities in the past few years. Diamond as a mineral was actually first synthesized in the laboratory in the 1950s but commercial production has been primarily limited for decades to very small crystals used in the abrasives industry. Today, sizable gem quality diamonds are being produced in dozens of factories around the world and their popularity is growing rapidly as they become more and more affordable. In this article we will discuss the two growth methods, each quite different from the other, for producing lab grown diamonds: Chemical Vapor Deposition (CVD) and High Pressure High Temperature (HPHT).

How HPHT Diamonds are Made

The first synthetic diamonds were made using the High Pressure High Temperature method. A massive diamond press was designed to enable immense pressures and temperatures like those found in below the crust of the Earth, in order to turn carbon in the form of graphite into diamond.

HPHT Press

Pressure on the order of 1 million PSI and temperatures in the range of 1500 degrees Celsius serve to melt a metal catalyst and the graphite creating an environment where carbon atoms will migrate to a diamond seed plate in the growth chamber. The carbon atoms from the mixture bond to the carbon atoms in the lattice of the seed plate, growing atomic layer by atomic layer.

This method of growth runs for the prescribed length of time, depending on the goals of the grow. The flask is then removed and broken open to reveal the diamonds created. The process is not stopped once it begins and any problems or deficiencies need to be cured by adjustments in subsequent grows. As we will see, the CVD method allows for more flexibility. But with that added flexibility, some quality issues can be introduced.

The modern presses used in HPHT growth weigh as much as 70 tons. As such, they are very expensive and factories containing multiple presses are very expensive to build, including requiring huge concrete foundations to support the enormous weight of this equipment. HPHT also consumes tremendous amounts of electricity in order to sustain these pressures and temperatures days or weeks at a time while the diamonds grow. Consequently, there are only a handful of factories utilizing this growth method.

The advantages of the HPHT method is that it can produce very pure and transparent diamonds without need for any secondary treatments. However, this comes at a higher cost than the CVD method.

Multiple HPHT Presses

How are CVD Diamonds Made

Instead of a pressure chamber, CVD diamonds are grown in a vacuum chamber. A hydrocarbon gas such as methane is put in the chamber containing a diamond seed plate. Microwave energy is used to heat the gas into a dynamic plasma which causes the carbon molecules in the gas to separate into carbon atoms and migrate to the diamond seed plate where they bond with its carbon lattice.

CVD Vacuum Chamber

Like the HPHT method, carbon atoms grow layer by layer on the seed over a period of days or weeks. But the CVD equipment is smaller and much, much lighter and less expensive. It is therefore a much lower economic bar to entry for a prospective grower, and consequently has attracted a much greater number and variety of growers. Most of the lab grown diamonds on the market today are CVD.

Another advantage that CVD has over HPHT is that because the growth environment is less hostile in terms of temperature and pressure, it is possible for the operator to look through a glass view pane and visually monitor the growth process. It is therefore possible to detect problems with the grow and stop the process to correct the issue. However, starting and stopping the process is a double edge sword for quality. While one growth defect might be eliminated, it can also introduce striations into the diamond lattice which can negatively impact the transparency of the diamonds produced. Too many interruptions and the resulting striation can make the diamonds appear a bit hazy.

While more energy efficient and less expensive to produce, CVD diamonds often need post-growth treatments in order to be marketable.

Multiple CVD Vacuum Chambers image courtesy of the GIA

Lab Diamond Treatments

Lab grown diamonds often have undesirable characteristics as a result of planned or unplanned imbalances in the chemical environment in which they are grown. Traces of boron and silica trapped within the carbon lattice, or voids in the lattice where a carbon atom should be, can give rise to problems such as a blue, brown or grey tinge. Flux and other growth remnants can also be included in lab diamonds, and can look similar to crystals or “carbon spots” in natural diamonds. In many cases these issues can be improved by post-growth treatments of one kind or another.

The two most common treatments for lab grown diamonds are HPHT and LPHT. Subjecting a lab diamond to additional HPHT treatment, even if originally grown by the HPHT method, can sometimes improve certain color tinges and other defects. CVD grown diamonds are commonly put into an HPHT press to correct issues with their original growth. Some lab diamonds can also be improved by a low pressure annealing referred to as LPHT or low pressure high temperature treatment.

There is debate in the market about quality differences between lab diamonds that are grown in such a way to need no post growth treatments (aka “AS Grown”), and those that require additional processing. On the one hand some argue that lab diamonds are a product of technology and it matters little what the specific process or processes are that are required to create the end product. For instance, they argue it makes little difference in the case of HPHT grown diamonds that are subjected to additional time in the HPHT chamber. The other side argues that diamonds grown without requiring additional treatment are a superior product, created by growers with the highest level of expertise using the state-of-the-art equipment.

There are other treatments as well including different types of irradiation to alter the atomic lattice and enhance or remove color in lab diamonds.

At this time in this relatively young market there seems to be a significant portion of shoppers that prefer As Grown lab diamonds.

Lab Diamond Quality –CVD vs HPHT

As we have seen, the two growth methods for producing lab diamonds are significantly different from one another. This gives rise to differences in certain quality factors between the two and which are fairly unique to lab grown diamonds in general. It is important to note that both methods are capable of producing outstanding quality diamonds.

CVD diamonds as we have seen, are prone to interruptions in the growth process. Since the operator can see the growth taking place in real time, there is a temptation to stop the process any time an issue is seen in order to pre-empt a problem. After the problem has been dealt with the reactor is restarted and the growth process continues. However, the atmospheric conditions within the reactor at startup are different than conditions when the reactor is fully up to speed. These fluctuations create striations in the growth patterns, much like the rings of a tree reflect the conditions during seasons with differing weather patterns.

CVD Growth Diagram

If there are too many interruptions in the growth process, the striae can have detrimental impact on the transparency of the diamond. The path of light rays transiting through the diamond is slightly distorted by a striation, and with too many the diamond can begin to look cloudy. This is something not normally noted on a typical laboratory report so it is critical to look for this when physically inspecting the diamond. Because the effect can be subtle, it often requires a trained eye to detect it. Dealing with a knowledgeable and ethical merchant is therefore just as important when it comes to purchasing lab grown diamonds as it is for natural diamond in terms of getting everything from the purchase that you intend.

CVD grown diamonds can also have color issues – brown and gray are the most common. Though perfectly transparent and colorless diamonds can be grown by CVD, the fact that there are more growers using this method because of the lower costs, there are also more growers who lack the highest level expertise needed to grow fine material by this method. As a result, many CVD diamonds need post growth treatments to mitigate some of the issues arising from their original growth process. Many are put into an HPHT press, to pressure treat them, thereby removing certain negative characteristics.

One of the unique issues with CVD production is contamination with silica from leaching of atoms from the glass view window into the growth chamber, producing a gray color due to voids created by the silica atom. Gray is not easily removed by post growth treatment, and can often worsen the color.

HPHT diamonds are grown in one step that avoids the problems associated with striation due to mechanical interruption of the growth cycle. HPHT growth can reliably produce very pure, very transparent diamonds. But problems can still occur in HPHT due to imbalances in chemical composition of the growth medium. One of the most notable is a slight blue tinge often referred to as “blue nuance” due to traces of boron. Gray tinge is also an issue caused by graphitic inclusion in the carbon lattice.

HPHT Growth Diagram

Another issue which seems to be a factor among some shoppers is phosphorescence. This is a type of luminescence that is caused by exposure to UV or VV light and persists after the light source is extinguished. This is distinct from fluorescence which very few lab diamonds exhibit as most lab grown diamonds are Type II without measurable nitrogen content, which is the leading cause of blue fluorescence. The impact of fluorescence on a diamond’s appearance in everyday lighting environments is negligible to very slight in the majority of cases, yet it is a factor that impacts value. To what extent phosphorescence is an appearance factor is likewise questionable but it does seem like the market is also approaching it as a negative characteristic.

And no discussion of diamond quality, whether natural or lab grown, is nearly complete without discussing diamond cut quality . No matter how good the starting material is, if the diamond is not cut with precision it will not possess the attributes we expect in a fine gem. Make sure the information associated with any diamond for sale is adequate to ensure that the stone has been crafted for optimal light performance

Pricing of Lab Grown Diamonds – CVD vs HPHT

Lab diamonds are a really young market in terms of the mainstream jewelry industry. As such, pricing of lab diamonds is difficult to get a handle on, but one thing is clear; the price of a lab grown diamond is much, much less expensive than its equivalent in a natural mined diamond!

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There are many new producers who are using different growing methods and treatment techniques to create marketable quality diamonds. They have different cost structures and different business models, which gives rise to significant variations in their pricing practices. There are also a full range of qualities being released on to the market, and some of these quality factors are not conveyed through a diamond report. A prime example is a transparency deficit due to striation or strain, an issue largely associated with CVD diamonds. Thus, seemingly equivalent diamonds on paper can be of very different quality in reality and may therefore be priced very differently.

As we have seen CVD growth is done in reactors that are small and affordable relative to HPHT presses which are very expensive and extremely large and heavy. Thus, CVD have a built-in advantage in terms of cost. As such, many growers who are new to the industry have been trying their hand at lab diamonds, leading not only to increased supply overall but to a higher volume of lower quality lab diamonds being released onto the market at cheaper prices. This has caused a faster fall in lab prices generally than would otherwise have been seen.

HPHT growers have the greatest investment and generally the most expertise in the field. Their cost structure and their overall quality is such that HPHT diamonds are more expensive. Because of their higher production costs, they act as a support for the price of lab diamonds in general. It is not sustainable for them to produce diamonds below a certain price threshold. Meanwhile, it is possible that as prices drop, the marginal CVD producers will exit the business leaving only the more expert CVD growers producing better overall quality. This will tend to further stabilize lab diamond prices and lead to a more predictable market structure.

Currently, the best way to understand pricing is to sample a number of different websites and catalogues of available diamonds, being sure to consider many of the issues discussed above in evaluating the relative value of those offerings. Because some defects are not revealed from reading the laboratory report, it is of utmost importance to consider the quality and reputation of the merchant offering them. In particular, are they carefully vetting the diamonds they offer for a full range of potential problems? Do they have extensive experience in the diamond business and light performance in particular? Do they provide extra evaluation services like advanced imaging or videography? Do they have the diamonds in stock? All of these factors will contribute to the ultimate price of the diamond.

Environmental Considerations of Lab Grown Diamonds

There is much debate around the issue of sustainability and social and environmental responsibility of lab grown diamonds versus natural mined diamonds. While the lab growers want to be considered a ‘green’ industry, and some are doing important things to reduce their carbon footprint, the issue is anything but black and white. An enormous amount of electricity is consumed in growing diamonds. Some growers are accessing some or all of their power through renewable sources which is commendable and definitely the way forward.

From a standpoint of beneficiation of communities on a global scale, the lab grown industry falls very short of the natural diamond industry. Factories growing diamonds benefit a relatively small of group of scientists and technicians (and their investors), whereas the natural diamond mining industry provides a living to millions of people across the world, many of whom are among the neediest on the planet. Natural diamond mining provides access to education and health care to communities that would otherwise have few options.

With regard to the difference between CVD and HPHT grown diamonds, the question of which is more responsible is even more complicated. It is true that CVD is more energy efficient in terms of the equipment. But it may be that in the long run, more high-quality diamonds can be supplied to the market through HPHT bringing down the differential in energy costs. It also might turn out that the better financed HPHT producers will invest in more efficient ways to access renewable energy sources, also mitigating the differential. On the other hand, it might be that CVD ends up being the main growth method as that technology becomes more mature improving in quality and in its already higher energy efficiency.

Conclusion

Lab grown diamonds are rapidly becoming a mainstream part of the jewelry industry. While diamonds were first created in the laboratory in the 1950s and have been produced for abrasives and other industrial uses for decades, it is only in the last few years that gem diamonds have come onto the market in commercial quantities. Popularity among consumers has been rising rapidly as price points for nice quality diamonds have been moving steadily in the direction of affordability.

Two very different methods of growing diamonds now predominate the industry. HPHT and CVD. The former utilizes huge metal presses to create enormous pressure and temperature to turn graphite into diamond, and the latter uses a vacuum chamber and hydrocarbon heated in a plasma reactor to “rain” carbon atoms onto a seed plate. Both methods require a high-quality diamond seed for the atoms to bond with, growing the carbon lattice layer by layer over days or weeks.

Both HPHT and CVD are capable of producing high quality diamonds. And both are subject to defects from the growth process that sometimes require post growth treatment in order for the finished diamond to be marketable. Each method has distinct defects, which enable laboratories to identify them and sometimes give an indication visually which growth method was used.

There are more CVD diamonds in the market today than HPHT, owing to more producers using this less expensive method. And many CVD producers are taking shortcuts in the process to manufacture as many diamonds in as short a time as possible and/or using lower quality and less expensive seed plates, leading to more lower quality lab diamonds in the market. Most CVD diamonds require post growth treatments to make them saleable.

Pricing of lab diamonds in general is all over the map due to many factors. Differences in cost structure and business model by various producers gives rise to pricing differences, as do quality issues that might not be obvious from a laboratory report. Being aware of these issues and making sure to deal only with a knowledgeable merchant with a great reputation such as Whiteflash is the key to a brilliant diamond purchase. Shopping with a company with high level ethics and expertise is as important when shopping for a lab diamond as it is for a natural diamond in order to get the best quality and value for your purchase.

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*All images used in this article were provided courtesy of the IGI unless noted otherwise.