It affects the actual hair shaft, rather than the follicle or the skin on your scalp. It causes hard, stone-like bumps, or nodules, on the hair. In fact, "piedra" is the Spanish word for "stone." ...

Nodular acne, or nodulocystic acne, is severe, inflammatory acne that causes acne nodules to form under your ... (a doctor specializing in skin, hair, and nail conditions) to treat nodular acne ...