From what I have read, this is "Hutchinson's sign," which means it's likely melanoma (and advanced, at that). But then I have a couple of questions:
My spot isn't a "longitudinal band," like most of the literature describes. It's a rounded spot that is pretty much in the "left corner." It doesn't start at the proximal part of the nail (well, it starts near the lateral part, I guess) and doesn't continue all the way up the nail. All this may not actually mean much of anything and I could be kidding myself, but I'm wondering if this makes it less likely to be cancer? Or if Hutchinson's sign really mostly applies to longitudinal melanonychia?
Under a flashlight, it's a light greenish brown, but it looks darker in other lighting. 5-6 weeks ago, I can be pretty sure it wasn't there, as I was looking at all my fingers and toes then because I had learned about this disease while looking up something else.