The 'generic' subclass of the definite article treated in the pag. 112, section 220.127.116.11 of the Modern Written Arabic: A Comprehensive Grammar reads as follows
"it denotes a generic meaning مائدة من النحاس المحفور 'a table of engraved brass’, contrasting with other possible materials. Indeﬁnite phrases also occur in a similar sense, عوارض غليظة من خشب ‘rough joists of wood’, but here the intention is not generic but rather ‘made of some kind of wood’ with no particular contrast with any other possible material."
The second volume of V. Cantarino's Syntax of Modern Arabic Prose, the expanded sentence, remarks that it's used 'after a determinative' and examplifies it in page 274 with the sentences صليب من الذهب 'a cross of gold', سرير من خشب 'a wooden bed' or ثوب من الخرير الأبيض 'a dress of white silk'.
However, I cannot find anything similar taken into account in generalist theories, although it seems to be a clear exception.
First, I cannot make sense of the contradictory english wording, so I'd like to find an elaboration thereof using more technical linguistic terminology.
The relation between 'Definiteness vs. Uniqueness/Specificity' could be the crux of the matter.