13C relaxation in natural diamond

13C spin-lattice relaxation times in the laboratory frame have been measured for one type Ia diamond and one type IIa diamond. Spin-lattice relaxation times for polarization in a field of 47 kG are shorter than spin-lattice relaxation times associated with the depolarization in a field of 47 kG after dynamic nuclear polarization in a field of 3.4 kG. If the 13C magnetization is inverted after dynamic nuclear polarization, the spin-lattice relaxation time is shortened, but it is still longer than the corresponding value for polarization in a field of 47 kG. It is suggested that spin-lattice relaxation via paramagnetic impurities is the main spin-lattice relaxation mechanism. A qualitative explanation is offered for the observed relaxation times.