  We propose a high-sensitivity magnetometry scheme based on a diamond Raman
laser with visible pump absorption by an ensemble of coherently microwave
driven negatively charged nitrogen-vacancy centres (NV) in the same diamond
crystal. The NV centres' absorption and emission are spin-dependent. We show
how the varying absorption of the NV centres changes the Raman laser output. A
shift in the diamond Raman laser threshold and output occurs with the external
magnetic-field and microwave driving. We develop a theoretical framework
including steady-state solutions to describe the effects of coherently driven
NV centres in a diamond Raman laser. We discuss that such a laser working at
the threshold can be employed for magnetic-field sensing. In contrast to
previous studies on NV magnetometry with visible laser absorption, the laser
threshold magnetometry method is expected to have low technical noise, due to
low background light in the measurement signal. For magnetic-field sensing, we
project a shot-noise limited DC sensitivity of a few
$\mathrm{pT}/\sqrt{\mathrm{Hz}}$ in a well-calibrated cavity with realistic
parameters. This sensor employs the broad visible absorption of NV centres and
unlike previous laser threshold magnetometry proposals it does not rely on
active NV centre lasing or an infrared laser medium at the specific wavelength
of the NV centre's infrared absorption line.
