We explore theoretically the possibility of obtaining efficient light emission from Si-Ge semiconductor nano-onions. Our motivation stems from the strong luminescence recently observed from II-VI nano-onions, combined with the technological significance of the Si-Ge system. We calculate the electronic properties of a range of hypothetical Si-Ge nano-onions using the tight-binding method and determine the combination of. size,; strain, and materials which optimizes the production of visible luminescence. We predict that small (less than around 30 Angstrom diameter) Si-covered Ge nano-onions will be visible light emitters.