Rhinoceros horns don't contain ivory, which is a hard, white material found in animal teeth and tusks. A rhinoceros' horn consists of keratin, the protein found in hair and animal hooves, and deposits of calcium and melanin.
The horns of most animals have bony cores covered with a fairly thin layer of keratin, but rhinoceros horns are keratin throughout with small mineral deposits at their centers. Calcium strengthens the horn and melanin protects the core against damage from ultraviolet radiation. Because of the supposed medicinal properties of rhino horn and the beauty of carved horns, poaching has brought all five species of rhinoceros to the brink of extinction.