In the IPA [ j ] is known as the palatal approximant and classified as a consonant. This sound is the English yod like in you. However, before I studied IPA, I intuitively thought of this sound as a diphthong of [i] and [u] or ee-oo.
Is it incorrect to think of [ j ] as a diphthong of ee-oo?
Actually, my first response to pizza was not totally correct. [j] is the sound made at the moment of transition from the initial vowel sound to the second. In [j] itself, there is no [i]; nor is there [u] or [ə] or any other vowel.
So, in general terms, can we agree that the diphthong of /i/ and another vowel produces /j/?
In a case such as 'pretty_iris', we normally talk of a glide rather than a diphthong. We tend to reserve diphthong for something that can represent a full syllables either alone, as in I/eye, oh, or when preceded and/or followed by consonants, as in tie, ice, mice.
It's all a question of a snapshot of the dipthong. An example (involving other languages, but it may be useful to students of English) is the Paul Simon song 'Me and Julio down by the schoolyard'. Like most English speakers, I thought of 'Julio' as having three syllables, so there was no internal rhyme in the lyric, and the words didn't scan. But I was wrong: the line correctly fits the two syllables of the name to two notes in the tune, and the '-ly'- of 'schoolyard' is quite close to the medial consonant in 'Julio'.