User blog comment:Legionaire22/Not an Ending Blog/@comment-24174486-20120724200828

Phantoms' swords are described as being "monomolecular". Blades literally a molecule thick would have excellent cutting power, but would need to be made of a strong material in order to be able to cut through something without breaking. Even then, the blades would still be fragile due to being so thin, thus Shepard being able to break Leng's sword as well as being able to destroy Phantoms' swords with gunfire or explosives.

As to what they're made of? Based on about five minutes of searching, I'm going to have to assume they're made out of scifium. X3