The new method to make artificial diamonds is interesting.

"Researchers have pioneered a method to grow diamonds under low pressure and temperature, revolutionizing the traditional synthetic diamond production process and expanding possibilities for scientific and technological advancements." (ScitechDaily, Paradigm Challenging Discovery: Diamonds Grown Without High Pressure)

The traditional method of creating artificial diamonds is based on the high-pressure and high-temperature (HPTP) method. That method requires very high pressure, which makes this traditional process dangerous. The new method base is in crystal growth in the liquid metal alloy. In temperatures of 1025 degrees Celsius, and melted metal alloys, the composition of gallium, iron, nickel, and silicon makes it possible to create diamonds at room pressure. But the temperature is still very high. 

The diamond rain in Uranus and Neptune can be modeled on Earth. "Carbon can link into a crystal on giant, icy gas planets like Neptune and Uranus because of the ultra-high temperatures and pressures deep down in the atmosphere. These conditions break up hydrocarbons like methane, allowing the carbon atoms within to connect with four others and make particles of solid diamond." (Space.com, Diamonds Could Be Raining From The Sky on Far More Planets Than We Realized) 

The diamond rain forms when ultra-cold methane ice falls into an extremely high-temperature area in the lower atmosphere. That thing raises the temperature in that ice very fast. And it creates the impact wave, that turns carbon into diamonds. The high-energy plasma with the magnetic field pressurizer can bring those conditions in Uranus and Neptune to Earth. 

If there is an economical way that doesn't need very much energy to create the artificial diamonds that method can used to solve the carbon problem. If compounds like ionized carbon dioxide are shot against the wall at extremely high speed the sudden stop can release so much energy that the carbon will turn into a diamond. 

How diamond rain could be happening on Neptune, and affecting its magnetic field. (European XFEL/Tobias Wüstefeld) (Space.com, Diamonds Could Be Raining From The Sky on Far More Planets Than We Realized)

In the same way high power explosion in the carbon-rich gas can cause a situation, in their energy and pressure impact can reduce carbon, hit it against the chamber's walls that thing can form at least a small part of the carbon into diamonds. 

There is a theoretical way to make the diamonds at a low temperature. The idea is that the conditions on planet Uranus are copied into the system. The idea is that the carbon or methane ice will shoot against the layer in high pressure. That impact can create high energy levels in the hydrocarbon or carbon. The sudden stop in the high-speed hydrocarbon or carbon object can turn the carbon into a diamond. 

The modified fusion system can also make diamonds. The carbon bite that is put into the metallic ball can turn into a diamond if the temperature and pressure are high enough. The system can use magnetic fields and lasers to make those conditions that turn carbon into diamonds. 

In some models, the carbon bite will close in the droplet. That droplet can be metal like mercury, melted iron, or water. Then the system shots droplets through laser rays and high-power magnetic fields. Those magnetic fields will create needed pressure in the droplet same way as fusion reactors act. And the laser systems turn the temperature high enough. That kind of electromagnetic system can create very high pressure in the droplet, and the outcoming laser rays deny the droplet vaporization. 

https://www.sciencealert.com/diamonds-could-be-raining-from-the-sky-on-far-more-planets-than-we-realized

https://www.space.com/diamond-rain-atmosphere-uranus-neptune

https://scitechdaily.com/paradigm-challenging-discovery-diamonds-grown-without-high-pressure/