Eleven brilliant cut yellow and brown diamonds were studied by non-invasive spectroscopic methods and devices. At first, identification was proven by Raman spectroscopy according to presence of Diamond Raman band at ca. 1330 cm–1 in each sample. UV/Vis/NIR spectroscopy was used as a potential tool to determine the diamond type. All studied stones were classified based on the presence of N3 – zero phonon line at 415 nm attributed to diamond Ia type which is the most abundant type in natural diamonds. This type consists of N atoms surrounding a vacancy. It suggests that diamond structure contains domains of type IaB diamond. Consequently, on the basis of type was excluded synthetic origin of the samples because this type was already not prepared artificially. Using Raman and UV/Vis/NIR spectroscopy, defects in diamond structure were detected and HPHT treatment was also determined. The main observable defects include N-V0 at 575 nm, N-V– at 638 nm, N3 at 415 nm, and H3 at 503 nm. The presence of these defects can be main indicator in HPHT treated stones. Only two studied samples significantly displayed all of these defects attributed to HPHT treatment, but other samples also showed tracks of treatment. This gemmological investigation results in estimating the original most likely brown diamond colour and probably intermediate temperature during HPHT processing.