Lab-Grown diamonds are not simulants (Cubic Zirconia).
They are diamonds created by scientists in a lab from rough crystals using HPHT (High Pressure High Temperature) or CVD (Chemical Vapor Deposition).
In short: scientists re-create the environment and conditions that exist in Mother Nature. As a result, the diamond still have variation in color and clarity, just as the earth produces a range of colors and clarities.