The /ʃ/ sound is spelled with <sh>, <ti>, <ci> or <che>. The /ʒ/ sound is spelled with <si>, <su>, <zu> or <ge>.
This is a contrast between two alveolar fricatives, distinguished only by voicing. Even native speakers may have a problem, but the contrast is so rare that it hardly matters.
The mean density value is very low at 0.2%. The list makes 6 semantic distinctions, a loading of 60%.
Aleutian allusion Aleutians allusions Asher azure Confucian confusion Confucians confusions dilution delusion dilutions delusions mesher measure meshers measures ruche rouge
John Higgins, Shaftesbury, January 2018.
(With thanks to Petr Rösel of Mainz.)