Light is essential to appreciate crystals, their transparency, their charm, the flashes that light rays light up reflecting on the faces… but the electromagnetic waves with different wavelengths that penetrate the crystal also produce a multiplicity of physical effects – complex chemicals (absorption, refraction, birefringence,…). If the incident waves are in the field of visible light, the most evident effect is the colour: the crystals take on the most varied colors as a result of complex mechanisms of absorption of specific frequencies. But crystals reserve us other and unexpected surprises: if the incident waves have more energy than visible light (for example in the field of UV radiation), then other electronic excitation mechanisms take over and also minerals which are colorless or white take on bright and intense colors, and those that already had color can change completely and take on fantastic new hues. It is the phenomenon of luminescence, which lasts as long as the crystal is subjected to irradiation. Sometimes it persists for some time even after excitation: it is the phenomenon of phosphorescence. The colors that the mineral assumes when irradiated are indicative of its nature and of the chemical elements present in the crystalline structure.
