Hello,
These dark lines can be due to can be due to longitudinal melanonychia(presence of a pigmented stripe, usually brown or black, along the length of the nail bed in darker-skinned individuals,splinter hemorrhages or Beau’s lines(they are horizontal lines of darkened cells and depression).Without examination, it is difficult confirming a diagnosis and so I would suggest a dermatologist’s evaluation.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
On one nail the lines are vertical and run the length of the nail and on the other nail it started with a line but now half of the nail is solid dark purple.