The /3/ vowel is spelled with <er>, <ir>, <ear>, <ur>, and <yr> in myrrh. The /ʊə/ diphthong is spelled <our> and <oor>.
This is a contrast between a long vowel and a diphthong. The starting positions are fairly close, but there is a difference of lip-rounding, so it causes no problems for learners.
Interesting pairs include:
pearly poorly
Shirley surely
The density figure is 1.33%. The set makes 10 semantic contrasts giving a loading of 71.4%.
burn bourn burns bourns burr boor burrs boors gird gourd girds gourds myrrh Moor pearly poorly per poor Shirley surely spur spoor spurs spoors turd toured year you're
Credit: John Higgins