Last updated on April 10, 2021
Lab-grown diamonds are created using two different processes: high pressure, high temperature (HPHT) and chemical vapour deposition (CVD). While both methods produce synthetic diamonds, it is important to understand the differences between HPHT vs CVD diamonds and how these processes affect diamond quality.
What Are High Pressure High Temperature (HPHT) Diamonds?
Natural diamond crystals were formed deep underground where the conditions of extremely high pressure and high temperature were naturally found. That is why the first and the oldest technique to grow synthetic diamonds was mimicking nature with high pressure and high temperature (HPHT).
There are three techniques to grow HPHT diamonds: the belt press, the cubic press and the split-sphere press. The goal of each is to create an environment of high pressure and high temperature where diamond growth can occur (pressure over 1.5 million pounds per square inch and temperature above 1400 °C). Each process starts with a seed of a diamond material which is placed in carbon, the element all diamonds are made of and put under extremely high pressure and temperature. In these conditions, carbon melts and forms into a diamond around the seed.
The first reproducible HPHT lab-grown diamond was created using the belt press by General Electric on December 16, 1954. The diamond was tiny and heavily included, so it was not suitable for jewellery manufacturing. It is worth mentioning that even today most of the synthetic diamonds produced are not of gem quality. They are created for industrial purposes where tools and equipment of excellent properties (hardness, thermal conductivity, etc.) are used.
With the success of the belt press, HPHT technology has been advancing with years; and modern cubic and split-sphere press have been introduced. These were more efficient and allowed to produce larger gem-quality diamonds. However, the HPHT diamond growing process is very expensive and produces diamonds with mainly yellowish and brownish tints because they are exposed to nitrogen while forming.
HPHT diamonds often contain metal inclusions such as iron, nickel and cobalt, because these metals are used during the growth process, and sometimes they enter the diamond crystal. These metallic inclusions can help experts to identify them as lab-grown because natural diamonds rarely capture metals during their formation.
Outside of growing diamonds, the HPHT process is used to enhance the colour of natural stones. This treatment is effective for enhancing the colour of some rough or polished diamonds, turning them into colourless, yellow, orange-yellow, yellowish-green, green, pink and blue.
What Are Chemical Vapour Deposition (CVD) Diamonds?
Created in the 1980s, Chemical Vapour Deposition (CVD) is a more recent diamond growing technique during which a gemstone is grown from a hydrocarbon gas mixture, imitating how diamonds form in interstellar gas clouds.
During the CVD process, a thin wafer of a diamond crystal is placed in a vacuum chamber and heated to 800 °C. Then the chamber is filled with a carbon-rich gas, such as methane, and get ionized into plasma using lasers, microwaves or others. The ionization breaks the molecular bonds in the gas, and pure carbon gets deposited on the wafer of a diamond crystal (usually HPHT synthetic), growing a gemstone atom by atom, layer by layer.
The CVD diamond growing process has a couple of advantages over the HPHT process. High pressure is not needed, and temperature, while high, do not need to be above 1400 °C, meaning this process is much less costly.
The CVD method can produce colourless gems with no colour zoning. CVD diamonds never contain metal inclusions, but they often come with graphite or other mineral inclusions that are a result of their growth process.
HPHT vs CVD Diamonds
The main difference between HPHT vs CVD diamonds is the way they grow or their morphology. HPHT diamonds grow in cuboctahedron shape and have 14 growth directions, while CVD diamonds grow cubic and have 1 growth direction. In comparison, natural diamonds grow in octahedron shape and have 8 growth directions. These growth patterns are also the main way to tell mined and synthetic diamonds apart.
As for the quality of lab-grown diamonds, HPHT has generally been associated with more yellowish and brownish diamonds, while the CVD method is the first to create colourless gems. Nowadays, both techniques can produce a colourless and flawless diamond, and there is no difference except the morphology.
It is worth mentioning that information about which method of growing diamonds is better is quite contradicting. However, what is happening behind the scenes is that each company tries to push what benefits them the most. A CVD company will tell their customers to avoid HPHT diamonds and vice versa.