Hello. im 25yr old female. i do realize that on several nails there are permenant black lines appears. why and what is wrong with my nails? tq

Melanonychia: Brown or dark streaks are called melanonychia. This is normal especially in black or ethnic skin. However, there is a risk of having melanoma so if there are changes in the streak you should definitely see a doctor or dermatologist to make sure it's not suspicious for melanoma.

Unsure: Black stipes down one nail could be a subungual hematoma. Red or brown streaks could be splinter hemorrhages. Brown/black lines from the nail plate to tip are often seen in people with dark skin & are normal. Subungual Melanoma can cause nail ridges and dark colors of the cuticle. It usually begin in the nail matrix. It most often effects the thumbs or big toes. Get checked out when in doubt.