I agree itís a diode to clamp the reverse EMF from the coil when itís released. That limits the pulse going back to whatever is controlling the solenoid, possibly the ECU.
That said, some solenoids have the clamping diode built in. Try measuring the resistance across the diode in both directions. It should be high in one direction and low in the other. Then try the same across the coil terminals in the solenoid. Both directions will be low because of the coil windings, but one lower than the other. Iíd be interested to hear what you find.