Fluorescence is the emission of electromagnetic radiation (e.g., visible light, ultraviolet light, X-rays, and gamma rays) from a substance, stimulated by the absorption of incident electromagnetic radiation. The emission persists only as long as the stimulating radiation is continued. The terms fluorescence and photoluminescence often appear interchangeably, but gemologists commonly use the former to describe the visible light emission from ultraviolet (UV) excitation. In most cases the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. Diamonds containing no defects or impurities generally do not absorb visible and UV light (230 nm) and thus produce no color or fluorescence. But when impurities or defects (often referred to as color centers) are present, they may absorb visible and UV light to produce color, fluorescence, or both. In diamonds with multiple color centers, the fluorescence from one defect may even excite the emission from another center more efficiently than the external UV light. Conversely, the fluorescence from one color center may be absorbed by other defects, dramatically reducing the overall fluorescence. Phosphorescence commonly refers to visible light emitted by a diamond after the stimulating UV light source has been discontinued. While related to fluorescence, it is a relatively uncommon visual property of gem diamonds and not a focus of this study.