HPHT Lab Grown Diamonds

For centuries, the idea of creating diamonds has captured human imagination. Once it was understood that diamonds are a form of the single element carbon, scientists spent decades trying to replicate the process. General Electric successfully produced the first synthetic gem quality diamonds in 1953 using what is now known as the High Pressure High Temperature (HPHT) method. This breakthrough recreated the extreme conditions found deep within the Earth, where natural diamonds form.

Concurrent with the development of HPHT, a different method called Chemical Vapor Deposition (CVD) was developed using a vacuum chamber to create an environment in which a super-heated plasma is generated using microwave energy. While both methods are capable of producing gem-quality diamonds, their very different growth processes result in distinct characteristics that gemological laboratories can identify and evaluate.

In this article, we focus on HPHT lab grown diamonds, exploring how they are created, how they compare to CVD diamonds, and what sets them apart from both a technical and visual standpoint.

• How HPHT diamonds are formed and the science behind the process
• Key differences between HPHT and CVD diamonds
• The role of seed quality and growth process on final product
• Quality issues such as blue nuance (HPHT) and diminished transparency (CVD)
• Why HPHT diamonds are associated with higher crystal quality
• What buyers should know when choosing between HPHT and CVD

This guide is designed to give you a clear, practical understanding of HPHT diamonds, with a focus on the factors that actually impact appearance and quality.

HPHT lab diamonds are grown in huge steel presses that weigh many tons. The chamber inside these giant presses is capable of generating pressures of 5-6 GPa. This has been likened to the pressure of a 747 jet balanced on the tip of a finger! The pressure range required is similar to the pressure range at 150-190km beneath the crust of the Earth. Temperatures of 1300-1600℃ are required in the process, which is higher than the temperate range for natural diamond formation (1040- 1250℃) and provides fast growth.

Into the chamber is placed a diamond seed crystal, a carbon source such as graphite, and a metallic flux. As the pressure and temperature are raised to the required levels, the carbon atoms in the source material become liberated by melting into the flux and gradually migrate to the diamond seed plate. They begin forming layers on top of the seed plate which attracts them and acts as a template onto which the carbon atoms bond and grow in a lattice configuration.

The HPHT method, because of its nature, permits only a single run. That is, the seed and carbon source are prepared in advance with a timeline that aligns with the size diamonds desired. Larger diamonds take longer to grow. But there is no stopping and restarting with this method, and no way to visually monitor the growth in progress. Because of this and other factors, diamonds grown by HPHT are not as likely to experience fluctuations in the growth process which can introduce defects.

Since the seed plate is the template for the carbon atoms as they are laid down one by one, layer upon layer in the lattice, any defects in the seed plate will be mirrored in the resulting grown diamond. This makes the quality of the seed plate crucial to the quality of the final product, and this is true of both HPHT and CVD growth. Very pure diamond seed plates with minimal lattice defects are more expensive to acquire. Large growers with tremendous investments in factories housing these huge HPHT presses tend to use only the finest starting seeds. This can be a factor in the qualitative differences between diamonds grown by HPHT vs CVD. It is also an additional factor in the cost basis of HPHT grown diamonds, which are generally more expensive to produce.

According to GIA in their latest studies, “a diamond substrate (often referred to as a ‘seed’ in HPHT growth) is used to create the crystal blueprint from which the new diamond is created. The quality, size, and preparation of the substrate can have a significant impact on the resulting diamond.”

One of the few issues for HPHT grown diamonds from a gem standpoint is the occasional presence of too much Boron in the growth chamber which gives the resulting diamond a blue undertone, referred to in the trade as “blue nuance”. This is often a very vague hue that is not captured in the color grade and is usually not apparent from the face up direction. But as with natural diamonds in the yellow range, the body color is detectable from the side in clinical grading environments. The blue hue, to the extent there is body color, is arguably more visually pleasing than yellow or brown hues normally seen in natural diamonds. However, because blue coloration is so rare in natural diamonds, lab created diamonds with blue nuance are somewhat stigmatized in the market.

Because diamonds grown by the HPHT method are rigidly constrained by huge pressure on all sides during their creation they tend to have far fewer structural defects such as crystal strain, a characteristic which is common in CVD growth. This results in a more perfect crystal and significantly reduces a quality concern that comes with lab grown diamonds and which is not graded on most laboratory reports - namely transparency.

A transparency deficit, whether caused by light scattering inclusions or from atomic level defects, can result in haziness and diminish light performance, even in precision cut diamonds. Because crystal strain can result in an impediment for crisp internal light propagation, if severe enough it can make a diamond hazy. This issue is problematic for the consumer because transparency is not a characteristic graded or measured on most laboratory reports. And because it is an atomic-level defect, it does not factor into the clarity grade. Thus, it is natural for consumers to assume a VVS diamond is fully transparent, but that is not always the case. And since the effect is often subtle, it can take a trained eye to assess it accurately.

“In general, colorless and near-colorless HPHT-grown diamonds have low impurity concentrations and uniform pressure is applied during growth, resulting in high crystalline perfection and very weak or almost no strain levels” (GIA- Gems and Gemology Summer 2024)

Post-growth treatments of HPHT grown diamonds are possible, but typically not necessary. CVD grown diamonds, by contrast, usually require post growth treatment to remove brown body color. The presence of nitrogen in the growth environment improves the growth rate of diamonds in this method, but also results in body color that then needs to be removed. Interestingly, the most common post-growth treatment for CVD diamonds is a form of HPHT treatment. It is different from the HPHT growth process in that it does not add material, and it involves higher temperatures than HPHT growth. CVD diamonds are also sometimes treated with LPHT in a vacuum chamber using very high temperature.

HPHT “As Grown” requires no additional treatment, which is an advantage in that post-growth treatment may or may not be fully successful in removing brown or other odd hues. Other unintended consequences might also result from treatment.

The presses that grow HPHT diamonds require careful preparation as the process involves a single run and cannot be stopped and re-started. Therefore, perfecting the growth environment is crucial for a successful HPHT grow.

“Whereas CVD-grown diamonds can be created over a series of growth steps, HPHT-grown diamonds are produced in a single uninterrupted run, making a highly controlled environment much more necessary.” (GIA)

• More closely replicates natural diamond formation. The resulting crystals even share a striking resemblance to natural diamond rough.
• Few structural defects in the carbon lattice creating a diamond with very high transparency, alleviating problems with haziness not revealed on a laboratory report.
• Does not require extra post-growth treatments to de-colorize them.
• HPHT growers are more likely to use the highest quality seed plates because of their overall investment.

The highly controlled environment necessary to grow diamonds by the HPHT method, and the fact that growth is constrained by enormous pressure on all sides, results in exceptionally high quality crystals that are free from defects such as crystal strain that can impair transparency.

Blue nuance is still a possibility resulting from excessive boron in the growth chamber, but this issue is much less common today as formulas and procedures have evolved and improved.

Diamonds grown by the CVD method are far more plentiful in the market with many more small producers involved because the cost of entry into the industry is much more accessible. Due to fluctuations in this growth method and the ability to grow diamonds in stages, more defects such as crystal strain are introduced. Such defects can sometimes result in transparency deficits (haziness). This may not be obvious to a non-expert such as a typical consumer and is not measured or graded on most laboratory reports. Therefore, more scrutiny is required to rule out appreciable defects in CVD diamonds compared to HPHT As Grown diamonds.