How laboratory-grown diamonds are made.
by Julia Griffith FGA DGA EG
Let's dive into the creation of laboratory-grown diamonds...
Laboratory-grown diamonds are diamond. They share the same essential chemical composition and crystal structure. Therefore, laboratory-grown diamonds are just as durable and beautiful as natural diamond.
There are two main methods for synthesising laboratory-grown diamond; HPHT and CVD.
Let's learn more about these processes...
Diagrams of the equipment that produces HPHT and CVD laboratory-grown diamonds
HPHT stands for high temperature, high pressure. Due to the extreme temperature and pressures involved, this process could be likened to natural diamond formation.
We're talking 1300 - 1600°C (2370 - 2900°F) and 5-6 GPa of pressure - the equivalent pressure to 80 African elephants standing on your big toe. This is higher than what diamonds form under in the mantle of our Earth.
Crazy, right? Even then, this temperature/pressure combination is only able to create laboratory-grown diamonds thanks to a mix of metals used within the process. The metals act as a catalyst, a flux, and as a vehicle for transporting the carbon atoms.
Diagram of the reaction cell in HPHT laboratory-grown diamond synthesis.
The HPHT reaction cell
HPHT laboratory-grown diamonds are grown within a reaction cell. These contain three key ingredients: a 'diamond seed', which is a tiny HPHT laboratory-grown diamond crystal, a mixture of three metals to make a catalyst, and graphite as the carbon source.
Under the high temperatures, the metal catalyst will melt. This dissolves the graphite releasing free carbon atoms into the molten metal which transports them to the base of the reaction cell. Due to the high temperatures and pressures, the carbon recrystallises as laboratory-grown diamond. This occurs on top of the seed, which acts as a nucleation point for growth - allowing one crystal larger to form.
Note: The seed does not 'grow', but functions as a growth platform. The carbon bonds to it allowing larger crystals to grow rather than lots of little crystals (which producers do sometimes to grow the seeds themselves as well as industrial laboratory-grown diamond grit).
Multiple stones of the same colour and quality can be produced within one reaction cell. All thats required are more 'seeds'. The amount of seeds possible depends on the size of the reaction cell, which, in turn, is determined by the type of HPHT machine being used.
Several machines have been designed to synthesis laboratory-grown diamonds using the HPHT method. The equipment must be able to impart the extreme pressures required so are often massive.
The most common HPHT equipment is the 'cubic press' - a huge machine that utilises six hydraulic anvils to put pressure onto the reaction cell where the laboratory-grown diamond grows.
The cubic press was developed in China, who currently produce the majority of HPHT laboratory-grown diamonds.
CVD stands for 'chemical vapour deposition'. It is complete different to HPHT synthesis and can not be compared to the earthly formation of natural diamonds.
In CVD synthesis there is no pressure. In fact, the growth chamber becomes a vacuum depleted of air. The growth chamber is pumped full of hydrogen and heated with microwave energy to create a plasma.
The plasma is as hot as the surface of the sun. Methane gas is then added, which is stripped apart by the high enbergy-plasma.
Methane's chemical composition is CH4 - this is the source of carbon. The carbon deposits on top of 'diamond seeds' (again - laboratory-grown diamond NOT natural diamond seeds).
The seeds act as a platform for growth. The carbon atoms attach on top in the form of laboratory-grown diamond.
This process can grow individual crystals weighing 10 - 15ct within 3 to 4 weeks.
Diagram of CVD laboratory-grown diamond synthesis
CVD & Treatment
CVD laboratory-grown diamonds are commonly grown with a brown tint. This is due to internal strain. These crystals are subjected to heat treatments to reduce this colour.
The treatments are either HPHT or LPHT treatments.
In HPHT (high pressure, high temperature) treatment, the stones are placed into a HPHT press. This is the apparatus used in HPHT synthesis.
In LPHT (low pressure, high temperature) treatment, the stones are heated within the CVD chamber after growth.
Both HPHT and LPHT treatment relieves some of the strain inside the stones. This reduces the colour.
It is estimated that 75% of the colourless CVD laboratory-grown diamonds on the market have been subjected to one of these 'decolourisation' treatments.
Laboratory-grown diamond crystals grown by HPHT synthesis.
Did you hear about the laboratory-grown diamond made from Ranch seasoning? Ah, the exciting world of diamond synthesis. Check out this story in my YouTube video below. Click the subscribe button in the bottom right to subscribe to this YouTube channel.