Gemstone Fluorescence Under UV Light — LW and SW Reference

UV fluorescence is one of the most underutilized tests in field gemology, yet it is also one of the fastest and requires only an inexpensive UV lamp. When a gemstone is exposed to ultraviolet light, certain trace elements or crystal structure defects cause it to emit visible light — a phenomenon called fluorescence. The color and intensity of that emission varies by species and, critically, can differ between natural and synthetic specimens of the same stone.

Gemologists use two UV wavelengths: longwave UV at approximately 365 nm (LW UV, also called "black light") and shortwave UV at approximately 254 nm (SW UV). The LW response and SW response are often different and both carry diagnostic value. Natural ruby, for example, typically fluoresces strong red under LW UV due to chromium, which also enhances its apparent red color in daylight. Synthetic ruby grown by flame fusion often shows the same strong red, but certain synthetic rubies grown by flux or hydrothermal methods can show different intensities. By contrast, red spinel — a common ruby simulant — shows a distinctive orange-red under LW UV, helping to differentiate it from ruby even before a refractometer reading.

Diamond fluorescence is well-known in the trade: approximately 30% of gem diamonds show blue fluorescence under LW UV, and rare "over-blue" diamonds can appear hazy in strong daylight as a result. The fluorescence of synthetic diamonds grown by CVD or HPHT methods differs characteristically from natural diamonds and is one of the screening tests used by major gemological laboratories. Among colored stones, amber shows strong blue-white fluorescence under LW UV — a useful test to distinguish it from glass imitations and copal resin, which fluoresce differently.

This table covers all 130 species in the GemID database and is the most complete combined LW+SW fluorescence reference freely available online. The "Chelsea Filter" column shows the response through an emerald (Chelsea) filter, which transmits deep red and yellow-green and is used for detecting chromium-bearing stones.

See also: Full Reference Database  ·  RI Chart  ·  SG Chart  ·  Mohs Hardness Chart