Ẓāʾ

Ẓāʾ is the rarest phoneme of the Arabic language. Out of 2,967 triliteral roots listed by Hans Wehr in his 1952 dictionary, only 42 (1.4%) contain ظ. Ẓāʾ is the least mentioned letter in the Quran, only being mentioned 853 times in the Quran.

In relation to other Semitic languages

In some reconstructions of Proto-Semitic phonology, there is an emphatic interdental fricative, ṯ̣/ḏ̣ ([θˤ] or [ðˤ]), featuring as the direct ancestor of Arabic ẓāʾ, while it merged with ṣ in most other Semitic languages, although the South Arabian alphabet retained a symbol for ẓ.

In Classical Arabic, it represents a velarized voiced dental fricative [ðˠ], and in Modern Standard Arabic, it represents an pharyngealized voiced dental [ðˤ] but can also be a alveolar [zˤ] fricative for some speakers.

In most Arabic vernaculars ظ ẓāʾ and ض ḍād merged quite early. The outcome depends on the dialect. In those varieties (such as Egyptian and Levantine), where the dental fricatives /θ/ and /ð/ are merged with the dental stops /t/ and /d/, ẓāʾ is pronounced /dˤ/ or /zˤ/ depending on the word; e.g. ظِل is pronounced /dˤɪl/ but ظاهِر is pronounced /zˤaːhɪr/, In loanwords from Classical Arabic ẓāʾ is often /zˤ/, e.g. Egyptian ʿaẓīm (< Classical عظيم ʿaḏ̣īm) "great".

In the varieties (such as Bedouin, Tunisian, and Iraqi), where the dental fricatives are preserved, both ḍād and ẓāʾ are pronounced /ðˤ/. However, there are dialects in South Arabia and in Mauritania where both the letters are kept different but not consistently.

A "de-emphaticized" pronunciation of both letters in the form of the plain /z/ entered into other non-Arabic languages such as Persian, Urdu, Turkish. However, there do exist Arabic borrowings into Ibero-Romance languages as well as Hausa and Malay, where ḍād and ẓāʾ are differentiated.

In English, the sound is sometimes represented by the digraph zh.

Notes:

1. In Mauritania (Hassaniya Arabic), ض is mostly pronounced /ðˤ/ as in /ðˤħak/ ('to laugh'), from */dˤaħika/ ضحك, but /dˤ/ generally appears in the lexemes borrowed from Standard Arabic as in /dˤʕiːf/ ('weak'), from */dˤaʕiːf/ ضعيف.
2. In Egypt, Lebanon, etc, ظ is mostly pronounced /dˤ/ in inherited words as in /dˤalma/ ('darkness'), from */ðˤulma/ ظلمة; /ʕadˤm/ ('bone'), from عظم /ʕaðˤm/, but pronounced /zˤ/ in borrowings from Literary Arabic as in /zˤulm/ ('injustice'); from */ðˤulm/ ظلم.
3. In some accents in Egypt, the emphatic /dˤ/ is pronounced as a plain /d/.