Whats method that appropriate for measure color of gemstone?
Measuring gemstones is not a trivial matter and some have constructed special instruments and sample holders for the purpose. It is reasonable to measure both transmittance and reflectance to get an idea of the gemstone color. The most perceptually meaningful measurement would be one carried out with more typical illumination and viewing conditions. This could be accomplished by arranging the desired illumination environment and then measuring the light from the gemstone at various angles using a telespectroradiometer. The Gemological Institute of America has developed measurement techniques and published on the topic in the past. Perhaps an inquiry to them or a search for their publications would be helpful.
