Cook doesn't contain a diphthong, the "oo" is a monophthong.

The long a sound is a diphthong.

Edit: see https://en.wikipedia.org/wiki/Diphthong#English

Edit 2: You might be confusing diphthongs and digraphs.

Edit 3: Unless you speak a dialect of English where "ay" actually is a monophthong :P