Researchers from the Arizona State University and the University of Chicago have proven through experimentation that it's possible there are planets that could mostly be made out of diamonds.
There are several factors to the recipe of space creating a diamond exoplanet, and one of those is if the planet is orbiting a star that is rich in carbon - a necessary ingredient in a diamond's composition. Another component is water, followed by pressure and heat. Gravity could have caused the necessary pressure, which has left scientists thinking that there could possibly be many planets out there already in this correct kind of environment.
The researchers decided to put the theory to the test, and took a silicon carbide and placed it in a very high-pressure situation. Adding water, compression with diamond "anvils", and heat from a laser to try and replicate the conditions of a cabron-rich planet, they were able to successfully create diamonds and silica. If you are interested in reading more about this story, check out this link here for more information.