Q:

Usage of "noun of noun": mountain of an uncle, gem of a person

Mountain of an uncle, gem of a person, dude of an actor, dreamboat of a husband. Please explain this usage of "noun of noun".

A:

From the OED (formatting mine):

of, preposition
24. Between two nouns in sense-apposition. b. in the sense "in the form of".
  The leading noun is the latter, to which the preceding noun with of stands as a qualification, equivalent to an adjective; thus that fool of a man = "that foolish man, that man who deserves to be called 'fool'"; that beast of a place = "that beastly place".

Following that definition, if someone is described as a gem of a person, the leading noun in this expression is person, and gem of is equivalent to beloved; therefore, a gem of a person = "a beloved person, a person who deserves to be called 'gem'". The other expressions you listed follow the same pattern.

