Seborrhoeic keratosis. Close-up of a seborrhoeic keratosis (also known as a seborrhoeic or senile wart) in the armpit of a patient. This skin growth is caused by over-production of keratin. Keratin is a tough fibrous protein found in hair and the outer layer of skin. Keratoses range from flat, dark brown rough patches to small, wart- like protrusions covered with a greasy, removable crust. Seborrhoeic keratoses are mainly found on the trunk and usually occur in the elderly. They are harmless, but as they are unsightly can be removed by cryosurgery (destruction of tissue using extreme cold) or surgical excision.