Is there a word that means to see something but not be able to describe?
I think the word you're looking for is either ineffable or euphoria (or a combination of both).
INEFFABLE: /ɪnˈɛfəb(ə)l/ too great or extreme to be expressed or described in words.
Via: http://dictionary.reference.com/browse/ineffable
AND
EUPHORIA: /juːˈfɔːrɪə/ a feeling or state of intense excitement and happiness.
Via: https://en.wikipedia.org/wiki/Euphoria