We synthesize CdS/PbS -coated nanoparticles and investigate the spectra of their optical absorption and photoluminescence (PL). The experimental results can be explained by the quantum-confinement model in coated semiconductor nanoparticles. We also prepare gold-coated nanoparticles with a nonmetallic core Au 2 S by a two-step colloidal method, and observe the extraordinary optical absorption spectra of the coated samples. The results are consistent with a theoretical approach that includes electromagnetic resonance effects and the quantum confinement of the carriers in the thin gold shell layer.