Richard Nordquist, at About Education, supports the belief that there are unresolved difficulties in deciding on whether some usages should be considered examples of 'adjectives' or 'attributive nouns':
• "Webster's New International Dictionary . . . does not call every
noun capable of attributive use an adjective but some like cash, land,
mind etc. are labelled 'n(oun) often attrib(utive).' However, the
distinction between words that are 'n often attrib' and words that are
'adj' is not precise, as the editors themselves claim . . ..
Moreover, even one author may provide different explanations for
similar cases. Gove (1964:165), for example, considers the word zero
in zero modification an adjective in the light of its attributive
and predicative uses, despite the fact that it neither inflects for
degree nor admits adverbial modification. However, surprisingly
enough, for macaroni salad, apparently similar to the zero
modification example, he argues that there appears to be a 'strong
feeling' against macaroni as an adjective." (Isabel Balteiro, The
Directionality of Conversion in English: A Dia-synchronic Study. Peter
Lang AG, 2007)
I'd throw in a plastic spoon and a tantalum rod.
A deeper discussion, including the debate about whether 'gradience' should be accepted as a necessary concept when seeking to allocate POSs to words in given sentences, is given in Pastor Gómez' Nominal Modifiers in Noun Phrase Structure: Evidence from Contemporary English.. I think the views of Denison he quotes are particularly important:
Denison (2001) recognises that diachronically there is a movement of
some modifying nouns into the class of adjectives, and that there is
thus intersective gradience from noun into adjective by some nouns in
modifying position, such as fun and key. In the case of fun, for
example, it is used as a modifier in attributive and predicative
position (eg a fun home; that new game is fun [to play]). Denison
even cites an example with a superlative ending, funnest, pointing
out that this is 'a sign of full morphological adjectivehood'....
Denison argues that there is no simple switch from the noun into the
adjective category, but rather a series of transitions from one
category into the other.'
[Of course, the pure noun fun still exists happily.]