Moles occur when skin cells, called melanocytes, form a cluster instead of getting distributed evenly throughout the skin, according to WebMD. Melanocytes are the components of the pigment melanin, which gives skin its natural color.
HowStuffWorks explains that melanin is the natural pigment that gives color to the skin, hair and the eyes' irises. Melanocytes, found in the two upper layers of the skin, produce melanin. Melanocytes are typically spread evenly throughout the skin, giving the skin a natural color; however, exposure to the sun causes them to produce more melanin, leading to a darker color of the skin. Moles often grow on areas frequently exposed to the sun, such as the face, neck, chest, arms and hands.
The majority of moles are not harmful, notes WebMD. Most moles grow in early childhood and during the first 30 year of an individual's life. Moles that do not change over time are not a cause for concern. Those that appear different than other existing moles or those that appear after age 30 need to be checked by a dermatologist. It is important to have a doctor evaluate a mole if it shows changes in color, size, height or shape or if it itches, bleeds or becomes painful.