  There is significant interest in potential experimental tests of macroscopic
quantum effects, both to test potential modifications to quantum theory and to
probe the quantum nature of gravity. A proposed platform with which to generate
the required macroscopic quantum spatial superposition is a nanodiamond
containing a negatively charged nitrogen vacancy (${\text{NV}}^{-}$) centre. In
this review, methods to fabricate nanodiamonds containing ${\text{NV}}^{-}$
suitable for these quantum applications are discussed. The proposed probes of
the macroscopic limits of quantum theory are presented along with the spin
physics of the ${\text{NV}}^{-}$ centre relevant to those tests.
