If the mole is more than 8 inches in diameter, it poses more risk of becoming cancerous. Atypical nevi are moles that are larger than average (larger than a pencil eraser) and irregular in shape. They tend to have uneven color with dark brown centers and lighter, uneven edges. These moles tend to be hereditary. A fact sheet about moles and how some moles may be related to melanoma, the most serious type of skin cancer. Includes photos of differences between moles.