The word diphthongising is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 28
Highest Scoring Words With Friends™ Play In The Letters
Seven Letter Word Alert: (35 words)
dinging, dinting, dishing, dogship, gothish, hidings, hightop, hinging, hinting, hoggish, hosting, indigos, insight, insipid, nothing, opining, phoning, piggish, pinging, pinions, pithing, poising, posting, shindig, shining, sighing, signing, singing, sniping, spionid, spiting, spongin, stoning, tidings, tinging