Single crystals of dark-red MgB7 were grown from the elements in a Cu-melt. The crystal structure (Pearson symbol ol64; space group lmma; a = 10.478(2) angstrom, b = 5.977(1) angstrom, c = 8.125(2) angstrom, 2842 refIns, 48 params, R-1(F) = 0.018, R-2(I) = 0.034) consists of a hexagonal-primitive packing of B-12-icosahedra and B-2-units in trigonal-prismatic voids. According to the UV-vis spectra and band structure calculations MgB7 is semiconducting with an optical gap of 1.9 eV. The long B-B distance of 2.278 angstrom within the B-2-unit can be seen as a weak bonding interaction. The new Mg similar to 5B44 occurs beside the well-known MgB12 as a byproduct. Small fragments of the black crystals are dark-yellow and transparent. The crystal structure (Pearson symbol tP196, space group P4(1)2(1)2, a = 10.380(2) angstrom, c = 14.391(3) angstrom, 4080 refIns, 251 params, R-1(F)= 0.025, R-2(I) = 0.037) is closely related to tetragonal boron-II (t-B-192). It consists of B-12-icosahedra and B19+1-units. With a charge of -6 for the B19+1-units and a Mg-content of similar to 20 Mg-atoms per unit cell the observed Mg content in Mg similar to 5B44 is quite close to the expected value derived from simple electron counting rules. All compositions were confirmed by EDXS. The microhardness was measured on single crystals for MgB7 (H-V = 2125, H-K = 2004) and MgB12 (H-V = 2360, H-K = 2459).