Are black streaks under the nail a melanoma 100% of the time?

NO: Only a nail biopsy will know for sure.

Black Streaks: The answer is NO. They are not melanomas 100% of time. Infant only 1% of white skinned people have this. Even then it is the most common melanoma diagnosed in dark skinned people. There is no racial aviation in its occurrence. Usually seen between 40-70. Trauma may be a cause for this . sun exposure is a questionable factor for these melanomas. See your dermatologist or skin cancer doctor ASAP .