welcome to HCM!
Lines and other deformities or color changes are common, and most of them are not serious. This is particularly true if the line runs vertically, as in YOUR case . fungal infections of the nail will cause similar lines, although typically there is also more generalized deformity
or thickening of the nail as well. Another possibility would be a foreign body under the fingernail, which can look like a black line. . Finally, rarely, bleeding can show up under the nails as small dark lines called splinter hemorrhages AND since you are constantly stubbing it under chair so subungual haemrrhage can be a possibility.
melanomas occur on nailbed but in your case its is not likely because it hardly presents like a straight vertical line.
you must visit a doctor especially to rule out any fungal infection or trauma.
Isincerely hope this will help you!take care!