When testing a diode with the multimeter set on "Diode Check," a reading of 0.6 volt is obtained. This indicates that the diode being tested is a

Rectifier

Explanation:

when The reading on a multi meter set used to test a diode shows a reading of approximately 0.6 Volt, it simply means that the diode being tested is a either a silicon diode or a rectifier.

setting the multi meter set to " Diode check" is done when one wants to check for the voltage ( forward or reverse bias ) across the diode. the current  passed through the diode is between : 1-10mA. and a reading of 0.6 to 0.7 volts indicates that the diode is a rectifier or a silicon diode

plasma is the state of matter where the substance is ionized and has no definite shape or volume. the fact of being ionized makes the substance a good electrical conductor.