Longitudinal Melanonychia is lengthwise brown or black band in the nail secondary to pigment (melanin).  Just like the skin, the nail also contains cells called melanocytes that are capable of producing pigment.  If the melanocytes are stimulated by trauma (stubbing a toe, picking a cuticle repetitively), the cells can “wake up” and start producing pigment.  This pigment appears as a brown or black stripe in the nail.  Melanoma is a rare but serious type of cancer that can appear in the nail if the nail melanocytes begin to grow in an irregular fashion.