  Nitrogen-vacancy centres (NVs) in diamond are attractive for research
straddling quantum information science and nanoscale magnetometry and
thermometry. While ultrapure bulk diamond NVs sustain the longest spin
coherence times among optically accessible spins, nanodiamond NVs display
persistently poor spin coherence. Here we introduce high-purity nanodiamonds
accommodating record-long NV coherence times, >60 us, observed via universal
dynamical decoupling. We show that the main contribution to decoherence comes
from nearby nitrogen impurities rather than surface states. We protect the NV
spin free precession, essential to DC magnetometry, by driving solely these
impurities into the motional narrowing regime. This extends the NV free
induction decay time from 440 ns, longer than that in type Ib bulk diamond, to
1.27 us, which is comparable to that in type IIa (impurity-free) diamond. These
properties allow the simultaneous exploitation of both high sensitivity and
nanometre resolution in diamond-based emergent quantum technologies.
