We report on the structural and the optical properties of silicon nanocrystals embedded in a silicon-dioxide matrix with a varying content of the crystalline silicon phase. Cross section transmission electron microscopy confirmed the presence of small crystalline silicon quantum dots with size distribution of ∼ 25%. Using continuous wave and time-resolved photoluminescence spectroscopy we were able to distinguish between characteristics of the PL decay that are associated with quantum confinement effects, and PL characteristics that are affected by the environment. In particular, we have found that the lifetime of the upper singlet state and the singlet-triplet energy splitting originate from quantum confinement of the carriers in the individual nanocrystals. On the other hand, we have found that the lower triplet state life-time should be assigned to nonradiative processes that are affected the crystallites environment. Furthermore, the oscillator strength for the radiative transitions was found to be significantly weaker compared to that of direct gap semiconductors. As a result, we conclude that the efficient PL from silicon nanocrystals is due to exclusion of nonradiative channels that gives rise to very long nonradiative lifetimes.