We have grown a variety of isolated GaN nanowires using gas-source molecular beam epitaxy (MBE) and characterized their structural and optical properties. The nanowires have demonstrated a number of promising materials characteristics, including low defect density and high luminescent intensity. Well-separated nanowires formed spontaneously on Si(111) substrates after deposition of a thin AlN buffer layer. Metal catalysts were not used. X-ray diffraction indicates that the c and a lattice parameters are within 0.01 % of the lattice parameters of bulk GaN. Transmission electron microscopy (TEM) revealed the nanowires to be free of dislocations and stacking faults, although a GaN matrix layer growing at the base of the wires was found to have a high density of basal plane stacking faults. The room temperature photoluminescence (PL) intensity compared favorably with a free-standing, thick film of high quality GaN. Several features of the low temperature PL spectra also indicated that the nanowires had few structural defects or chemical impurities. Finally, electrical characterization of dispersed nanowires demonstrated that efficient electrical contacts could be made and that the resistivity of the nanowires was comparable to that of bulk material.