  Compact, high-precision inertial sensors are needed to isolate many modern
physics experiments from disturbances caused by seismic motion. We present a
novel inertial sensor whose mechanical oscillator fits on a standard one-inch
diameter optic. The oscillators achieve a Quality factor of over 600,000 and a
resonance frequency of 50\,Hz, giving them a suspension thermal noise floor
lower than all commercially available inertial sensors. The oscillator is
combined with a Pound-Drever-Hall based readout scheme that achieves a
displacement noise of 100\,f\msqrthz above 0.2\,Hz. We integrate the oscillator
and readout to make two inertial sensors. Of order n$g$ performance is achieved
in a broad band from 0.1\,Hz to 200\,Hz. Below 20\,Hz, the sensor presented
here offers comparable performance to the best inertial sensors available today
while being a fraction of the size. Above 20\,Hz, the sensor is, to the
author's knowledge, the best demonstrated in the literature to date for a
device of this style, with a self-noise floor of 0.1\,n$g$\sqrthz. The
excellent performance of the sensors across the relevant seismic frequencies,
vacuum compatibility, and compact size make it a prime candidate for
integration into sophisticated seismic isolation schemes, such as those used by
gravitational wave detectors.
