Irradiation bombards gems with gamma rays, neutrons, or electrons to displace electrons or create color centers in the crystal lattice. It produces or intensifies colors in materials like blue topaz (most commercial blue topaz), some diamonds (fancy colors), and certain tourmalines. A subsequent low-temperature heat treatment often stabilizes the color.
The process is controlled in specialized facilities. Colors are usually stable, though some may fade under prolonged strong sunlight or heat. Identification uses spectroscopy to detect radiation-induced absorption bands and microscopic features. All irradiated gems must be disclosed; they typically sell for less than naturally colored equivalents.