Clark's nevus, also known as a junctional nevus, is a type of mole that typically appears as a dark brown or black spot on the skin. It is usually smaller than a pencil eraser and has a smooth, even surface. Clark's nevi are typically benign, but they can sometimes develop in to a type of skin cancer called melanoma, so it is important to monitor any moles on the skin for changes and to see a dermatologist if you notice any unusual changes.