角质 spans biology (keratin in skin/nails/hair) and skincare advertising. The compound 去角质 (qù jiǎozhì, 'remove keratin') is the standard Chinese term for chemical or physical exfoliation — you'll see it on every Chinese-market beauty product.
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角 = horn, 质 = substance/material. Literally 'horn-substance' — the same protein that makes animal horns also forms human nails and hair.
