21. Ultraviolet light
Measuring between 100 to 380 nanometres, ultraviolet light is too short for the eye to see without any protection. However, shortwave UV lights (SWUV) are used in gemstone testing as they can expose gemstone treatments and assembled stones.
As certain gemstones emit bright and unusual colours when exposed to UV light, one can ascertain the layers of diffusion-treated gems, oils used as fillers in emeralds and similar gems, and assembled stones by having them exposed to SWUV.
