A focusing gamma-ray telescope based upon perfect crystals was originally proposed for astrophysical observations in 1990s. However, the high angular resolution/low integral reflectivity of the perfect crystals employed made these types of optics less attractive for astrophysics than grazing incidence multilayer mirrors. Unlike astrophysical applications, the long-range detection of nuclear materials requires modest angular resolution and narrow energy
bandwidth (or several narrow bands) at specific gamma-lines emitted from nuclear materials. A development of large high gamma-reflective Ge and Si mosaic crystals makes the use of a diffraction mosaic gamma-telescope for long-distance detection of nuclear materials, as well as for astrophysics, a distinct possibility. This paper describes gammareflectivity results for Ge mosaic crystals in the range of 100-300 keV and a gamma-ray focusing telescope design based on the Ge and Si mosaic crystals with mosaicity in range of 1-15 arc minutes both for long-range detection of nuclear materials and astrophysics.