Argon in spark-gap tests

I am hardly an expert in Tesla Coil design or operation, as I am new to
experimentation with high voltage for that matter.  But in the tests I
have done, I have seen time and again that inert gasses actually appear
to be excellent conductors when energized with a high voltage discharge.
This would account for the spark following the flow of argon in your
spark-gap tests.  The discharge follows the path of least resistance,
and an inert gas apparently has far more conductivity than the ionized
air within the points.