Candace Zenon - October 16, 2023
Unveiling the Science of Lab-Grown Diamonds: Insights from a Chemical Engineer
I recently had an amazing opportunity to travel to India and visit several lab grown diamond factories. The experience was rewarding in more ways than one, however I came away with lots of questions about all I had seen and heard. Some of the offices we visited were large and lavish, others were small and spare. All of them extolled the virtues of their product offerings and enticed us with budget friendly bling, but it’s the science behind the stone that I’m interested in.
There are two predominant processes for producing lab grown diamonds, HPHT and CVD. With the HPHT (High Pressure High Temperature) Process, a diamond seed is placed in graphite powder within a reactor with a catalyst under conditions meant to simulate how diamonds form in nature. The diamond seed and the catalyst are sort of like the sourdough starter everyone was crazy about in 2020. They kick off the process and the diamond starts to form around the seed. This process can lead to the presence of metallic impurities within the diamond. These may be visible under magnification which can help identify a diamond as HPHT or if the metal impurities conduct electricity, then the diamond would test as Moissonite on a pen tester. Additionally, HPHT diamonds can be grown in the presence of Boron which can cause a bluish tint known as “blue nuance” which is undesirable. Diamonds with this bluish tint are also more likely to “fail” a diamond test with a pen tester. The growth rate for HPHT diamonds is much faster than CVD with viable crystal size reached in as little as 5-10 days. HPHT diamonds can typically achieve higher color as grown versus CVD diamonds.
CVD diamonds are formed at much lower pressures and the process is generally thought to be safer. A diamond seed is placed in a reactor with a heating element at temperatures up to 2000oC. A plasma, superheated gas, typically consisting of Hydrogen and Methane is introduced to the chamber and the Carbon is selectively deposited onto the diamond seed in even layers. The temperature and gas flow rates must be closely monitored to prevent buildup of graphite within the reactor. It’s very important that the Carbon concentration not exceed 5%. Because of this low concentration, the growth rate is very slow, averaging 1mm of growth over two days. The seeds used in this process can be made from an HPHT diamond or a CVD diamond and they are reused multiple times. Most CVD diamonds are subjected to post-growth annealing with HPHT treatment to remove any undesirable color. Size is a limiting factor with CVD diamonds. We will talk more about crystal growth in a moment.
Interestingly, there are a ton of patents being issued related to CVD diamond growth. Diamond as a material for use in the semi-conductor and electronics industry is driving nearly all of this technical development. Just a couple of interesting facts; for the same surface area, diamond can carry 5000 times the current of silicon, for the same thickness diamond can withstand 30 times the voltage of silicon. With traditional semiconductor manufacturing, various elements are added to silicone to adjust the properties needed for a specific application. Hilariously, these elements are called dopants and the process is referred to as “doping the silicon”. Anyway. The same process is being experimented with in CVD diamond production which has the side benefit of producing varying colors! For example the addition of Boron during the CVD growth process increases the electrical conductivity of the diamond and, of course, turns it blue! As far as I can tell, the published research on achieving colors in CVD diamonds is purely related to achieving desired electrical properties, but perhaps there will be applications for jewelry as well.
One of the major limitations with replacing silicon with diamonds in the electronics industry is the size that can be attained through the CVD process. There are some reports that forming a growth plate using two different types of seeds may be a solution. Diamond seeds may be formed from diamond or from electropositive elements, depending on the properties desired in the finished diamond. Electropositive elements, what is that. Just think of a battery that has a positive and negative terminal. The positive terminal has electropositive elements and the negative terminal has electronegative elements and the flow of electrons back and forth makes the battery work. So, if you combine seeds from diamonds and seeds from electropositive elements into a growth plate and coat it with something like silicone, perhaps you can grown larger diamonds that could benefit both the jewelry trade and the electronics industry.
One of the major arguments with lab grown diamonds is that they are a greener alternative. I think it’s important for people to be well informed before passing that claim on to the customer. Most of the CVD lab grown diamonds production uses methane and hydrogen as reactor gases to facilitate Carbon growth on the diamond seed. Methane is natural gas and is a co-product of oil production. Methane gas is also recovered using processes such as fracking that many consumers find objectionable. Hydrogen is also produced as a byproduct of oil refining or in some countries from coal. Hydrogen can be produced from renewable sources as well, albeit in smaller quantities. The demand for methane gas is expected to increase in the future as the demand for clean gases for electronics manufacturing increases. This includes CVD diamond growth as well. I found two companies, Aether and Sky Diamonds that use recycled CO2 as the carbon source for their CVD diamonds. If I were supplying stones from one of these sources to my customer, I would confidently say it was a sustainable alternative. I think it’s important to have frank discussions with your supplier in all other cases.
Seeing the same science applied to two different industries is fascinating! We have to keep in mind that while the reactor kinetics may be the same for growing a CVD diamond from factory to factory, the individual setpoints will be different. Kind of like the difference in taste between Dunkin and Starbucks. They’re both using coffee beans but each one tastes a little different and they’re not going to tell you exactly how they achieve the flavor. The more informed we are as jewelers, the better questions we can ask our suppliers to best serve our customers.