Fluorescence

A visible glow produced by ultraviolet light, including the ultraviolet present in daylight. In low-iron rubies, classically from Mogok, red fluorescence adds a lit-from-within intensity that iron-rich stones cannot match; it is part of what the market pays for. Laboratories also read fluorescence patterns as evidence, helping separate deposits and flag some treatments. Ask to see a fluorescent stone in plain daylight, where the effect actually lives.