We present the results of a comprehensive photoluminescence study of defect centres in single SiO2 nanoparticles. We show that the photo-physical properties of the luminescent centres strongly resemble those of single dye molecules. However, these properties exhibit a large variability from particle to particle due to the different local chemical environment around each centre of each particle. This variability provides new insight into the complex photo-physics of single quantum emitters embedded into a random chemical environment. Moreover, a better understanding of the fundamental mechanism of the photoluminescence of defect centres in SiO2 structure is paramount for their application as white-light sources, non-toxic labels for bio-imaging, or for combining them with metallic and semiconductor nanostructures.