It’s a couple millimeters wide, faintly brown colored, and runs from root to tip. I suppose an injury might cause it, but I don’t remember suffering any injury to that thumb, and besides, I’ve actually lost two other nails and they eventually grew back normally.
A pigmented band (or bands) extending from the base to the tip is a normal variant that is relatively common in darker skinned people. There’s some melanin producing cells at the site where your nail is being made and the pigment is incorporated and moves with the nail so it makes a stripe.
On the other hand, if it showed up quickly, it could be a sign of a potentially serious problem, especially if you are light skinned. If this is the case, I’d recommend getting it checked out.
PC
The “potentially serious problem” that PC refers to is malignant melanoma.
Hmm. I’ve noticed once in a while little patches of a non-pink color (usually white, sometimes light pink) appear near the root of the nail. THey are never more than 3 millimeters on a side, and the continuous growth of the nail means that they are off within two weeks or so.
The appearance of these thigns doesn’t appear to be linked to impacts on the nailbeds. I do a lot of manual (digital 
) work, and it doesn’t have anytthing to do with trauma to the finger.
I guess that the same mechanism that produces a little offcolor for me once in a while is always on for yoiu.
I get the spots sometimes too. And I am half Filipino, but I only noticed the stripe fairly recently.