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 T S0 which is above the critical temperature, a plateau is observed in the resistivity versus temperature curve ͑T͒ taken at zero magnetic field. When a magnetic field of B = B SN ͑N =1,2, ... ,5͒ is applied, the plateau moves to low temperature with the thermoresistivity maximum located at T SN ͑N =1,2, ... ,5͒. The ͑B͒ curves, measured at different temperatures around T SN , intersect in the-B plane at the field of B = B SN. By tuning B SN from 0 to 5 T, a series of plateaus in the-T plane and the corresponding intersections in the-B plane are observed. The intersections quadratically chain up in the-B plane, separating the superconducting from the insulating region. The thermoresistivity maxima exponentially group up in the-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.