The systematics of the suppression of superconductivity with increasing magnetic field in boron-doped nanocrystalline chemical vapor deposition diamond is studied in a broad temperature range. At the temperature of TS0 which is above the critical temperature, a plateau is observed in the resistivity versus temperature curve ρ (T) taken at zero magnetic field. When a magnetic field of B= BSN (N=1,2,5) is applied, the plateau moves to low temperature with the thermoresistivity maximum located at T SN (N=1,2,5). The ρ (B) curves, measured at different temperatures around TSN, intersect in the ρ -B plane at the field of B= BSN. By tuning BSN from 0 to 5 T, a series of plateaus in the ρ -T plane and the corresponding intersections in the ρ -B plane are observed. The intersections quadratically chain up in the ρ -B plane, separating the superconducting from the insulating region. The thermoresistivity maxima exponentially group up in the ρ -T plane, thus defining a phase fluctuation zone. The phase boundary, composed of the intersections and separating the superconducting states from the insulating state, is shown to be a generic consequence of granularity. © 2010 American Institute of Physics.