The /3/ vowel is spelled with <er>, <ir>, <ear>, <ur> and <our> in adjourn. The /ɔɪ/ vowel is spelled <oi> or <oy>.
This is a contrast between a long vowel and a diphthong. The starting positions are fairly close but it does not seem to be a problem for many learners. However the merging of these two sounds, so turning earl and oil into homophones, is a characteristic of an old-fashioned form of the New York accent, as represented in some gangster fiction, some of P.G.Wodehouse's novels, and the musical Guys and Dolls.
Interesting pairs include:
The density figure is 4.46%. The set makes 17 semantic contrasts giving a loading of 48.6%.
adjourn adjoin adjourned adjoined adjourning adjoining adjourns adjoins averred avoid bird buoyed burr boy burrs boys burr buoy burrs buoys curl coil curled coiled curling coiling curls coils curly coyly curt quoit earl oil earls oils early oily earlier oilier earliest oiliest first foist firsts foists furl foil furled foiled furling foiling furls foils inverse invoice learn loin learns loins purrs poise turd toyed verse voice versed voiced verses voices
John Higgins, Shaftesbury, December 2009