Enhanced stones are stones that have been treated to improve their color, clarity, finish, strength, or other characteristics. Some common enhancements are heat-treatment, irradiation, coating the surface, filling cracks, oiling, surface diffusion (coating the surface then applying heat), bleaching, dyeing, etc.
Nearly all gemstones available today have been enhanced to bring out their best color or to strengthen them. For example, an accepted industry practice in the polishing process for sapphires involves heating the stones to bring out their color. This process simply extends what nature began, since it is heat and pressure that give gemstones their color in the first place.