  First order coherence measurements of a polariton condensate, reveal a regime
where the condensate pseudo-spin precesses persistently within the driving
optical pulse. Within a single 20 $\mu$s optical pulse the condensate
pseudo-spin performs over $10^5$ precessions with striking frequency stability.
The condensate maintains its phase coherence even after a complete precession
of the spin vector, making the observed state by a definition a spin coherent
state. The emergence of the precession is traced to the polariton interactions
that give rise to a self-induced out-of-plane magnetic field that in turn
drives the spin dynamics. We find that the Larmor oscillation frequency scales
with the condensate density, enabling external tuning of this effect by optical
means. The stability of the system allows for the realization of integrated
optical magnetometry devices with the use of materials with enhanced exciton
$g$-factor and can facilitate spin squeezing effects and active coherent
control on the Bloch sphere in polariton condensates.
