  Yttrium orthosilicate (Y$_2$SiO$_5$, or YSO) has proved to be a convenient
host for rare-earth ions used in demonstrations of microwave quantum memories
and optical memories with microwave interfaces, and shows promise for coherent
microwave--optical conversion owing to its favourable optical and spin
properties. The strong coupling required by such microwave applications could
be achieved using superconducting resonators patterned directly on
Y$_2$SiO$_5$, and hence we investigate here the use of Y$_2$SiO$_5$ as an
alternative to sapphire or silicon substrates for superconducting hybrid device
fabrication. A NbN resonator with frequency 6.008 GHz and low power quality
factor $Q \approx 400000$ was fabricated on a Y$_2$SiO$_5$ substrate doped with
isotopically enriched Nd$^{145}$. Measurements of dielectric loss yield a
loss-tangent $\tan\delta = 4 \times 10^{-6}$, comparable to sapphire. Electron
spin resonance (ESR) measurements performed using the resonator show the
characteristic angular dependence expected from the anisotropic Nd$^{145}$
spin, and the coupling strength between resonator and electron spins is in the
high cooperativity regime ($C = 30$). These results demonstrate Y$_2$SiO$_5$ as
an excellent substrate for low-loss, high-Q microwave resonators, especially in
applications for coupling to optically-accessible rare earth spins.
