Fausset's Bible Dictionary
An ingredient of the anointing unguent (Exodus 30:34). Shechecleth means literally, "a shell or scale", the horny cap of a shell. The operculum or "cover" of the strombus or "wing shell", which abounds in the Red Sea, is employed in compounding perfume, and was the medicine named blatta Βyzantina or unguis odoratus in the middle ages. Pliny (H. N. 32:46) and Dioscorides (Matthew Med. 2:11) mention a "shell", onyx , "both a perfume and a medicine"; "odorous because the shell fish feed on the nard, and collected when the heat dries up the marshes; the best kind is from the Red Sea, whitish and shining; the Babylonian is darker and smaller; both have a sweet odor when burnt, like castoreum ." The onyx "nail" refers to the clawlike shape of the operculum of the strombus genus; the Arabs call this mollusk "devil's claw." Shell fish were unclean; hence, Gosse conjectures a gum resin.
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Fausset, Andrew R. Entry for 'Onycha'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/o/onycha.html. 1949.