They are generally harmless and are called pigmented birthmarks because of their various shades of coloring from light brown or black to bluish or blue-grey. Most moles or nevi appear after birth and are rarely a health threat.
Many people decide to remove moles for cosmetic reasons.
Once gone they rarely grow back. Even so, it s a good idea to check large, dark brown moles regularly. If there is a sudden and significant color change, if they bleed or become ulcerous, or if there s loss of hair on or around a 'hairy' mole, it s a good idea to seek medical advice. The formation of pigmented birthmarks has to do with melanin , a biological colorant found in the tissue of all living things. If there s a complete absence of melanin in skin, fur, feathers, or the eyes, the condition is known as albinism .