gems, gemology, basic gemology, advanced gemology, gem testing, spectroscopy, chemistry, chemical analysis, corundum, X-ray diffraction

The use of photoluminescence imaging for gemstone characterization is reviewed, considering both fluorescence and its time-delayed counterpart, phosphorescence. Luminescence results from the excitation of atomic impurities and defects by an external source. Fluorescence can be excited by ultraviolet, visible, or infrared light, or even X-rays. Fluorescence to long-wave UV light is a characteristic included in diamond grading reports issued by major gemological laboratories. This article provides a comprehensive overview of the principles, mechanisms, and characteristics of luminescence that create the impressive and memorable glow of gemstones. Although diamond is the focus, a variety of colored stones and pearls are reviewed as well. This article is intended to foster a deeper appreciation of the complexity necessary to understand these natural wonders.