Excision of a solar keratotic lesion on a woman's arm under local anaesthetic. A solar keratosis is a small wart-like red or flesh-coloured growth that appears on an exposed part of the skin. It is caused by over-production of keratin (the tough protein that is a major component of the outer layer of skin). Such growths are a result of overexposure to the sun over a period of years. They may rarely develop into skin cancer and therefore they are always removed.