Answer:
Made of Silicon.
Step-by-step explanation:
A diode is a semiconductor device use in mostly electronic appliances. It is two terminals device consisting of a P-N junction formed either in Germanium or silicon crystal.
Diode can be forward biased or reverse biased.
When a diode is forward biased and the applied voltage is increased from zero, hardly any current flows through the device in the beginning.
It is so because the external voltage is being opposed by the internal barrier voltage whose value is 0.7v for silicon and 0.3v for germanium.
If you measure 0.7 V across a diode, the diode is probably therefore made of Silicon.