In diamonds, this generally refers to a process of Fracture Filling, invented by Yehuda, and used to improve the appearance of a diamond. Such treatments are fairly durable, but not invincible, so I do not recommend them for engagement rings. This treatment should be disclosed by the seller, and they are not graded by GIA or the major labs because the treatment is not permanent (in the case of a strong heat source like a jeweler’s torch used for repairing prongs, etc.)
There is nothing wrong with selling these, as long as the treatment is disclosed, and complete explained, for full understanding of the risks in buying a stone with these treatments.
Other diamond treatments, like laser drilling to burn out impurities… or heat and pressure treatments to change or improve color, are lasting and permanent, so they are less risky. These treatments should also be disclosed at time of sale, as well. GIA and the major labs do grade such diamonds, and disclose the treatments on their reports.