On·y·cha n.
 1. An ingredient of the Mosaic incense, probably the operculum of some kind of strombus.
 2. The precious stone called onyx. [R.]

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 Onycha
    a nail; claw; hoof, (Heb. sheheleth; Ex. 30:34), a Latin word
    applied to the operculum, i.e., the claw or nail of the strombus
    or wing-shell, a univalve common in the Red Sea. The opercula of
    these shell-fish when burned emit a strong odour "like
    castoreum." This was an ingredient in the sacred incense.