Q:

Are abstract nouns also considered as adjectives? (parts of speech)

Courage (abstract noun) is a great virtue.
Though I am quite sure it cannot be considered as an adjective also, but just want to clear the doubt once and for all.Can a part of sentence be labelled under more than one part of speech?

A:

No, abstract nouns and adjectives are two different things.  Anger is an abstract noun, while "angry" is and adjective. "Length" is an abstract noun, while "long" is an adjective.
While specific words can be categorized into different parts of speech depending on how it is used in a sentence, specific parts of a particular sentence can only be categorized as one part of speech.  For example, take the word "love". 
A mother's love is eternal. (here love is an abstract noun)
I love pizza. (here love is a verb)

