The process of making HPHT diamonds is similar to the natural diamonds under the earth. HPHT diamonds process starts with a small diamond seed placed into pure carbon. Then, a diamond seed is exposed to intense pressure and heat. This pressure and temperature produce the HPHT diamonds as beautiful as natural diamonds.
Source:- https://abddiamonds.medium.com/what-goes-inside-the-process-of-hpht-diamond-creation-1631afa019ec