Materials are important, but so are the configurations of surfaces. Lots of
flat surfaces which are exactly parallel will exacerbate echo. Some
non-parallel surfaces will break up the resonance. You'll never see
parallel walls in a concert hall, but you'll see them all over restaurants
and convention halls.

So I would say that if you are building a room where "people might be
sitting and conversing", consult an architect with experience in acoustics.

Edward (who values quiet restaurants)