Metallic zinc nanoparticles (NPs) of 5-15 nm in diameter, formed in silica glass (SiO(2)) by Zn ion implantation of 60 keV, showed a strong ultraviolet absorption peak at around 4.8 eV, which has been assigned as the surface plasmon resonance (SPR) of Zn NPs, and another small peak at 1.2 eV, which has never been reported before. To identify the origin of the 1.2 eV peak, the correlations of thermal stability between the two peaks and Zn NPs were evaluated under annealing both in a vacuum (pure thermal stability) and in oxygen gas (thermal oxidation stability). The well-correlated stability between the 1.2 eV peak, the 4.8 eV peak and Zn NPs indicates that the 1.2 eV peak is not ascribed to radiation-induced defects but to the Zn NPs. The 1.2 eV peak can be ascribed to an SPR of Zn NPs in SiO(2), because the peak satisfies the criterion of the SPR of metallic NPs. Since the 4.8 eV peak is also expected to satisfy the criterion, Zn NPs in SiO(2) have two SPRs at 1.2 and 4.8 eV.