The word diphthongise 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: 23
Highest Scoring Words With Friends™ Play In The Letters diphthongise
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Seven Letter Word Alert: (56 words)
deposit, dieting, dingies, dishing, dogship, dopiest, editing, edition, eighths, ghosted, godsent, gothish, heights, hidings, highest, hightop, histone, hogtied, hogties, hoisted, hosting, ignited, ignites, indepth, indigos, indites, inosite, insight, iodines, iodites, ionised, iphones, nightie, phished, phonies, pigeons, pingoes, pithing, pointed, poising, posited, posting, shindig, shoeing, sighted, spinode, spionid, spiting, sponged, stipend, thighed, thiones, thonged, tidings, tiepins, topside