The /ɔ/ vowel is spelled <or>, <oor>, <oar>, <au> and <aw>. The /ʊə/ diphthong is spelled <oor>, <our>, <ure>, <ewe> in crewel and <ue> in cruel. The contrast saw/sewer would not be minimal if one treats sewer as a two-syllable word. There is a difference of stress in the pair inshore/insure. For many speakers the word lure contains a palatal glide which would make in non-minimal in its various contrasts.
Although this is a contrast between a long vowel and a diphthong, the starting positions are fairly close and confusions can easily occur. Some RP speakers, especially speakers of an old-fashioned variety, conflate these phonemes, using /ɔ/ throughout.
Interesting pairs include:
The density figure is 3.23%. The set makes 33 semantic contrasts giving a high loading of 82.3%.
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Credit: John Higgins