Nitrogen-vacancy (NV) centers in millimeter-scale diamond samples were produced by irradiation and subsequent annealing under varied conditions. The optical and spin-relaxation properties of these samples were characterized using confocal microscopy, visible and infrared absorption, and optically detected magnetic resonance. The sample with the highest NV− concentration, approximately 16 ppm (2.8×1018 cm−3), was prepared with no observable traces of neutrally charged vacancy defects. The effective transverse spin-relaxation time for this sample was T2∗=118(48) ns, predominately limited by residual paramagnetic nitrogen which was determined to have a concentration of 49(7) ppm. Under ideal conditions, the shot-noise limited sensitivity is projected to be ∼150 fT/√Hz for a 100 μm-scale magnetometer based on this sample. Other samples with NV− concentrations from 0.007 to 12 ppm and effective relaxation times ranging from 27 to over 291 ns were prepared and characterized.