Duolingo marked "Elle fait un mur de pierre" wrong, demanding "Elle fait un mur de pierres". But then the following sentence had "un mur en pierre". I'm pretty familiar and comfortable with the "noun + en + singular" construction to describe something in/made of a certain material, but why doesn't a noun in the singular work in "noun de noun"? Google seems to show tons of hits for "mur de pierre"
Quoting from the Banque de dépannage linguistique : Les prépositions introduisant une matière:
Les prépositions de et en permettent toutes les deux d’introduire un complément de matière. L’emploi de en était autrefois critiqué, mais il est maintenant passé dans l’usage courant.
So, both un mur de pierre et un mur en pierre are both fine nowadays. It is also perfectly fine to have pierre in the singular or in the plural here. The singular referring to the substance in itself, the plural referring to the substance being divided into fragments. So un mur en pierres and un mur de pierres are also fine.
But we would not use the plural with any sort of substance. I would find weird to write une bague en ors, even though the ring would be made of white, yellow and pink gold. A little less weird to write une robe en/de tissus, the use of the plural indicates straight away that the dress is not made of one single material. With tissu/s, both en and de are used but I would think that en is more frequent, but de is not wrong.
Un mur en béton, un mur de béton are both used, but I don't expect béton would be used in the plural in this case, even if there are different kinds of concrete. I expect the use of the plural in the case of describing the material being used much depends on what the eye can perceive at fist sight.