The /aɪ/ diphthong is spelled <iCe>, <i>, and <y>. The /ɔɪ/ diphthong is spelled <oi> or <oy>.
This is a contrast between two closing diphthongs with similar starting positions. It is unstable in some accents of English, notably Irish and "Mummerset" (exaggerated West Country rural). It is not a problem for EFL learners.
Interesting pairs include:
The density figure is 7.52%. The set makes 31 semantic contrasts giving a loading of 52.5%.
aisle oil aisles oils ally alloy allied alloyed allies alloys allying alloying bide buoyed bile boil buy boy buying buoying buys boys divide devoid fie Fowey file foil filed foiled files foils filing foiling grind groined I'll oil imply employ implied employed implies employs implying employing isle oil isles oils kind coined kine coign kite quoit kites quoits lighter loiter lightered loitered lightering loitering lighters loiters line loin lines loins mild moiled mile moil miles moils pies poise pint point pints points ply ploy plies ploys rye Roy sigh soy tide toyed tie toy ties toys tied toyed tying toying tile toil tiled toiled tiles toils tiling toiling try troy vice voice vices voices vied void vile voile
John Higgins, Shaftesbury, December 2009