What is the Moho Discontinuity made of?
The Mohorovicic Discontinuity marks a change in the composition of rocks, from basalt in the crust to heavier silicates (peroidotite, dunite) in the mantle.
The precise nature of the Moho is not fully known. But the fact of a composition change is evident from seismic waves traveling faster below the boundary than above it. See here: