And some diapasons are made of wood, not metal. Generally, the stopped diapasons I have encountered had a more "flutey" sound, but that depends on what the voicer wanted the rank to sound like. Pipe sounds and names are all over the place.

This is one of the most confusing named stops of all time, Casavant, isn't it!

However, organ flutes and strings are derived from diapasons and should be voiced so that combining a stopped flute and string will produce a diapason tone as well, but at a different level than the diapason.

Playing a diapason with a stopped flute emphasizes the roundness of a diapason, playing one with a string puts adds more of an edge to the diapason

They can serve to add richness to the organ chorus.