In British English the old genitive forms in -en expressing "made of X" are still in use (alongside the basic noun) for some words, notably golden, woollen , wooden, earthen, and (rarely) oaken but not for much else - though I have heard silvern once or twice. In the game of cricket, if a batsman fails to make a run in an "over" (a group of six deliveries (pitches), after which the bowlers change ends) then that is described as a maiden over, which I guess makes maiden the genitive form of maid.
are those really genitives and not some kind of suffix that just makes adjectives from nouns? I ask because that's how it is in German. We have the words "golden", "silbern" ("silvern"), "ehern" (from iron), "hölzern ("wooden") etc. as well and they are no genitives. And "golden" in German denotes "made of gold" as well as "having a golden colour".
Well, NibblyBits, you're not the first person to propose purging all foreign elements from the German language and culture, but this is something that ended very badly! If you think Deutschland should be some idyllic anachronism, full of buxom St. Pauli Girls and blond-headed, lederhosen-wearing clockmakers, with no Italian Owls, you can just keep this xenophobic nonsense to yourself. It's not like that in Germany anymore!
I believe the correct grammar is, ''Duo has teeth made of gold.'' And, that Duo's answer is incorrect English grammar. Why? Well ...
Duo's gold teeth are still a metal of gold The teeth can be returned to a state of being just gold, e.g. melted-down.
And so, ''... are of ...''.
Whereas, ''Duo's paper books are made from wood.''
(Trees > Wood > Paper > Books). And, as these books cannot be returned back to wood the correct word structure to use is, ''... made from ...''.
the tooth = der Zahn. the teeth = die Zähne, etc.
For individual words a dictionary (physical or online) and https://translate.google.com are good, easy sources. You'll get better, more detailed responses to questions that aren't so easy to find instant answers on your own.