Thesis (Ph.D.)--Chulalongkorn University, 2005

A novel transflectance technique using an infrared microscope was introduced for spectral acquisition of loose and mounted faceted gemstones. The transflectance technique exploited total internal reflection phenomenon within the faceted gemstone for the spectral acquisition. The incident radiation is coupled to the gemstone via the table facet while the transflectance radiation is collected by the built-in 15X Cassegrain objective. A novel accessory for adjusting the reflection plane of the faceted gemstone was constructed and was utilized with the infrared microscope. The technique is nondestructive, requires no sample preparation, and has short analysis time. FTIR spectra of faceted gemstones, synthetic gemstones and gemstone simulants collected by diffuse reflectance, specular reflection, and tranflectance techniques were compared. The same spectra features with better spectral quality as those from the well-accepted diffuse reflectance technique are observed by the transflectance technique. Unlike the diffuse reflectance spectra, the transflectance spectrum was not affected by the diamond arrangements. The technique can be employed for direct spectra acquisition of mounted gemstone without taking the gemstone out of the jewelry setting. Moreover, an individual gemstone on the complex jewelry setting can be selectively measured. Infrared absorption bands unique to the chemical compositions, impurities, and treatment processes of the gemstone were discussed. The observed transflectance spectra can be exploited for gems classification.