First, the planner starts with a diamond crystal. The planner decides what is going to happen with the crystal. One stone, two, three, what shape?
The marker then marks where the diamond needs to be sawn or cleaved.
If the diamond is to be cleaved, a kerf (small groove) is cut into the crystal with a laser. Then, some extremely skilled person gets to whack it with a mallet.
The crystal is more likely to be sawn, these days.
The next step is bruting which rounds out the stones and gives the initial cone shape to the pavilions. In this step, two diamonds are used to shape each other. This can be done by hand or machine.
Next, the diamond is blocked. The table plane is cut, and the pavilion mains and crown facets are placed. The brillianteers then add the remaining facets and do the final polishing.
This video shows most of the process: