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On the quality of /ḍ/ and /ẓ/ cf. Steiner, R. 1977, The Case for Fricative-Laterals in Proto-Semitic. New Haven with copious references, including references to the native Arab(ic) grammatical tradition.
The article "Ḍād" by Kees Versteegh in K. Versteegh et al. (eds.), Encyclopedia of Arabic Language and Linguistics, Leiden - Boston, vol. 1, 2006 is the most recent review of the topic.