A nail; claw; hoof, (Hebrews sheheleth; Exodus 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.

Noah Webster's New International Dictionary of the English Language

1. (n.) An ingredient of the Mosaic incense, probably the operculum of some kind of strombus.

2. (n.) The precious stone called onyx.