A reflectivity meter can be used with gemstones having of any refractive index,even over 1.81.UV LW/SW Light for Testing Luminescence/FluorescenceGems that emit visible light after exposure to short-wave or long-wave ultraviolet radiation,they are said to be luminescent,or more specifically,flourescent.A gem is phosphorescent if the luminescence continues after the UV light source has been removed.This phenomenon of fluorescence and/or phosphorescence is the result of UV radiation absorbed by impurities or structural defects within the crystal structure,resulting in an oscillation of electrons between energy levels,and transmission of visible light.
　　Fluorescence produces vivid colors when an ultraviolet or invisible light source is directed at a gemstone.Where did this term come from?In the middle of the nineteenth century,Professor George Stokes was experimenting with sunlight and the mineral fluorite.He was in a darkened room and admitted sunlight through a small hole in a window shutter.When sunlight fell on a col-orless fluorite specimen,he noticed the mineral displayed a bright blue color.Stokes coined the term fluorescence to describe the phenomenon he observed from the fluor-spar or fluorite,which he said was analogous to opalescence observed in opals.Therefore,fluorite is the basis for the word fluorescence but not all fluorite fluoresces!The fluorescence can be a bold green,orangishred,or whitish blue,and vary in intensity.Fluorescence may be unpredictable because some gems will have no reaction to the UV light source.
　　In rough material,a cleavage break may already be obvious or it can be determined by giving the specimen a tap with a hammer.Rough diamond is often cleaved and then cut into shapes.Cleavage is not possible to observe in fashioned gems unless an internal imperfection can be observed or there is an accidental blow struck along a cleavage direction and the gem breaks.