Irradiation of Diamond with accelerated electrons produces desirable coloration of the Crystals.
The HPHT Process can transform certain types (IIa) of brownish diamonds to white or near white color. Other diamonds (Type Ia) can be transformed to greenish, yellowish, or yellowish orange colored diamonds.
Nitrogen-vacancy centers are typically produced from single substitutional nitrogen centers by irradiation followed by annealing at temperatures above 700 °C. Irradiation produces lattice vacancies, which are a part of N-V centers.
Diamonds with large fracture inclusions can be processed to enhance its clarity. If the fracture opens up to the surface, glass like material with reflective index similar to that of the diamond (2.42) can be injected into the crack to diminish its appearance. Certain fracture in the diamonds can practically dissappear after this process to visually improve its clarity.
A single concentrated carbon spot in a diamond due to its multiple internal reflection can give an impression of heavy inclusions in other wise nice brilliant cystal. These spots can be bleached out by drilling microscopic tunnel and then injecting solvents to dissolve.