Melanoma is the least common type of skin cancer. Still, it is the most deadly because it can spread if not caught early. Melanoma symptoms include a change in the shape, size, or color of a mole, but melanomas can also look like a bruise that doesn't heal or a dark streak under a finger or toenail. Excess sun is linked to melanoma risk. However, melanomas can also occur on skin not exposed to the sun (such as inside the mouth).