What’s the Difference?
Lab-grown diamonds are created using two different processes: high pressure, high temperature (HPHT) and chemical vapor deposition (CVD). HPHT vs CVD diamonds – let’s see the difference between the two methods and their effect on 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 jewelry 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 for years, and modern cubic and split-sphere presses 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 identify them as lab-grown since natural diamonds rarely capture metals during their formation.
Outside of growing diamonds, the HPHT process is used to enhance the color of natural stones. This treatment is effective for enhancing the color of some rough or polished diamonds, turning them into colorless, yellow, orange-yellow, yellowish-green, green, pink and blue.
Laboratory-grown CVD rough diamond (left), laboratory-grown HPHT rough diamond (middle) and natural rough diamond (right). Image courtesy of GIA.
The main difference between HPHT and 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 colorless gems. Nowadays, both techniques can produce a colorless 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.
Written content courtesy of Diamond Buzz