Unlike the horns of cattle and antelope, which sheath a bony outgrowth of the skull, rhino horn is entirely epidermal. It is built from keratin filaments packed in a matrix, with calcium and melanin deposits that stiffen the core and harden the outside against ultraviolet damage. The horn sits on a roughened area of the nasal bones but is not fused to them.
The practical result is that horn grows continuously through life and is worn to shape by rubbing and feeding. Dehorning, used as an anti-poaching measure, removes most of the horn above a growth zone; the horn regrows over subsequent years, so the procedure must be repeated. Black rhinos have two horns, the front usually longer.
The common claim that horn is chemically the same as fingernail is close but imprecise. Both are keratin, but rhino horn has a distinctive internal structure and mineral content, and its purported medicinal properties have no clinical support in either case.