Answer:
Antiquities is a noun. The adjective form is antique (which is also a noun). Examples:
Noun: The antiquities of the native people should be returned to their care.
Adjective: My mother gave me her antique rocking chair.
Noun: When I purchased this item at the tag sale I had no idea that it was an antique.
The word antiquities is sometimes used as an adjective, for example Antiquities Dept. or Antiquities Curator, but the word is most often a noun.