Metal? I kid, of course.
Possible alloys used:
Titanium was a hard metal often used as defensive plating on war engines. - Titanium
Duranium was a tough metal used in military-grade plating. It was stronger than titanium but less resilient than impervium. - Duranium
Ceramisteel was a material used in vehicle hull construction. - Ceramisteel
These pages don't offer a lot in terms of proof, nor can I find a specific reference to what the Falcon is made of.
The Millennium Falcon does have a Deflector Shield which provides some protection from physical and energy attacks.
HAN (to Chewie)
We're coming up on the sentry ships.
Hold 'em off!
Angle the deflector shields while I charge up the main guns!
However, as I recall in the Force Awakens, Rey is attempting to get the shields online while she is crashing about learning to fly. This suggestions that the Falcon has significant structural integrity, because as you mentioned, it does withstand several major collisions before she gets the hang of flying and is able to activate the shield.
