Sebaceous naevus. Light micrograph (LM) A sebaceous naevus is a rare type of birthmark that can be found on the face, neck, forehead, or scalp. While it can appear anywhere on the head, it most often occurs on the scalp. Though it's technically classified as a hair follicle tumor and is associated with other conditions, a sebaceous naevus is benign. They are well-circumscribed, smooth, hairless "plaques" composed of sebaceous glands and do not usually cause any issues other than cosmetic concerns. Magnification: x100 when printed 10cm wide.