Absorption, scattering and fluorescent properties of several different types of nanodiamond samples are measured to characterize them for various applications. Two different methods, spectrophotometry and photothermal spectroscopy were used to measure absorption properties of nanodiamonds suspended in aqueous solutions. Photothermal spectroscopy provides the advantage of measuring absorption of photoactive nanodiamonds with high-sensitivity. Spectral fluorescence properties of nanodiamond samples were studied using a commercial spectrofluorometer and a home-built inverted microscope integrated with a light-sensitive imaging spectrograph. Characteristic fluorescence spectrum of nitrogen-vacancy defects in single diamond nanocrystals was obtained using the light-sensitive instrument.