What are the types and methods of synthetic diamond production, and how to choose?

Industrial diamonds, also known as synthetic diamonds, are primarily produced through three methods: High-Pressure High-Temperature (HPHT), the explosive method, and Chemical Vapor Deposition (CVD). The following article will briefly introduce these three diamond production methods, as well as the characteristics of the diamonds they produce, to help customers understand and identify the best choice for their needs.

1. High-Pressure High-Tamperature (HPHT)

Diamonds are composed of a single known element—carbon (C), the same as graphite. The difference lies in their structure: graphite has an SP² structure with a two-dimensional molecular plane (CN=3) in a trigonal arrangement, while diamonds have an SP³ structure, forming a three-dimensional molecular lattice (CN=4) with tetrahedral coordination. Therefore, by applying sufficient pressure and temperature—providing enough energy—and using an appropriate catalyst, graphite can be converted into diamond. Typically, converting graphite into diamond requires a high temperature of 1200-1500°C combined with 50,000-70,000 atmospheres of pressure. There are two mainstream production methods for this process: the two-zone press and the six-zone press. However, since the six-zone press reduces both production costs and increases efficiency compared to the two-zone press, over 90% of high-temperature, high-pressure diamonds are now produced using the six-zone press method.

2. Explosion method

There are two methods for producing diamonds via the explosive synthesis technique:

1. Direct Synthesis: In a sealed space, explosives are ignited, causing a gas explosion that propels carbon atoms to collide with each other at high pressure in an oxygen-deprived environment. This rapid compression of the carbon atoms from the explosive itself forms “explosive soot.” The diamonds produced by this method are extremely small in size with a large specific surface area, which makes them prone to absorbing impurities. This direct synthesis method is primarily used to produce single-crystal nanodiamonds.
2. Ultrasonic Shockwave Impact on Graphite: Another explosive method involves using ultrasonic shockwaves to impact graphite, converting it into micron or nanodiamonds almost instantaneously. In this process, graphite is mixed uniformly with copper powder and shaped into rods using isostatic pressing. The rods are then sealed inside a steel tube that contains a vacuum layer and surrounded by several tons of explosives. Once the explosives are detonated, the steel tube is compressed from one end to the other. Inside, the graphite experiences extreme conditions, with temperatures reaching several thousand degrees Celsius and pressures ranging from 200,000 to 300,000 atmospheres. In just a few microseconds, the graphite is converted into diamond. The diamonds produced through this method are polycrystalline and contain many defects, typically resembling popcorn or potato shapes. As a result, this process is mainly used to produce polycrystalline nanodiamonds or micron-diamonds.

3. Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition, also known as CVD Diamond, is a more modern method of cultivating diamonds.

In this process, a vacuum chamber is filled with carbon-containing gas, typically methane. A carbon plasma forms at the base of the chamber and continually deposits onto a carbon substrate at the bottom. The crystallization process can take several weeks, and the resulting crystal typically has rough, black edges and often appears brown. However, this brownish color can be removed through heat treatment. Diamonds synthesized using the CVD method are usually brownish or light gray, but adding trace amounts of nitrogen or boron to the vacuum chamber can yield yellow, pink-orange, or blue diamonds. It is also possible to grow colorless diamonds through this method, though it requires a longer growth cycle. Most diamonds produced through the CVD method are of the Type IIa classification, known for their purity. Many colorless diamonds on the market grown by CVD were originally brown crystals that have undergone HPHT annealing to remove the color. CVD-grown diamonds and HPHT-grown diamonds exhibit different gemological properties, with CVD diamonds often having higher clarity and fewer dark inclusions.