  We use magnetic-field-dependent features in the photoluminescence of
negatively charged nitrogen-vacancy centers to measure magnetic fields without
the use of microwaves. In particular, we present a magnetometer based on the
level anti-crossing in the triplet ground state at 102.4 mT with a demonstrated
noise floor of 6 nT/$\sqrt{\text{Hz}}$, limited by the intensity noise of the
laser and the performance of the background-field power supply. The technique
presented here can be useful in applications where the sensor is placed closed
to conductive materials, e.g. magnetic induction tomography or magnetic field
mapping, and in remote-sensing applications since principally no electrical
access is needed.
