The original method is High Pressure High Temperature (HPHT) and is still the most widely used method because of its relative low cost. It uses large presses that can weigh a couple of hundred tons to produce a pressure of 5 GPa at 1,500 degrees Celsius to reproduce the conditions that create natural diamond inside the Earth. The second method, using chemical vapor deposition or CVD, was invented in the 1980s, and is basically a method creating a carbon plasma on top of a substrate onto which the carbon atoms deposit to form diamond.
A coating treatment applies a thin film of synthetic diamond to the surface of a diamond simulant. This gives the simulated diamond certain characteristics of real diamond, including higher resistance to wear and scratching, higher thermal conductivity, and lower electrical conductivity. While resistance to wear is a legitimate goal of this technique, some employ it in order to make diamond simulants more difficult to detect through conventional means, which may be fraudulent if they are attempting to represent a simulated diamond as real.
A diamond simulant, sometimes misleadingly called a simulated diamond is not the real thing. A simulant is an imitation. Examples of diamond simulants include glass, sometimes called paste, white sapphire, white spinel, YAG (Yttrium Aluminium Garnet), moissanite, and cubic zirconia.