When a spark plug is removed, a brownish stain that looks like a sign of combustion gas flow is often seen at the caulked portion of the metal shell.

This stain is the result of oil particles suspended in air adhering to the surface of the insulator due to the high voltage. It does not affect spark plug performance.

Mechanism of corona discharge
The high voltage applied to the spark gap is also applied to the area between the center electrode and the metal shell, causing an insulation breakdown of the air at the gap (a) between the insulator and the metal shell. The phenomenon is called a corona discharge. The generated corona discharge develops toward the terminal nut. This last process is the pale blue corona discharge that may be observed in dark conditions.