In this study, we investigate several impurities in bulk GaN crystals grown by the sodium flux technique. In our system, black GaN is in general associated with oxygen, sodium, and molybdenum concentrations in excess of 1019 atoms/cm3. Here, we report on the correlation of high levels of oxygen with opaque crystals. Samples that contain oxygen concentrations above 1019 atoms/cm3 are highly absorbing regardless of the presence of other impurities. Experiments with partially submerged seeds indicated material grown mainly above the nominal liquid surface was more transparent and had a generally lower impurity concentration, especially oxygen. In optically transparent regions, concentrations of oxygen, sodium, and carbon were as low as 7 × 1016, 2 × 1016, and 5 × 1016 atoms/cm3, respectively, while the concentration of molybdenum was below the detection limit.