Ive had a little purple crescent on my toenail for years now. I never really stressed about it, doctor says it's a birthmark but I don't remember it when I was young. Nothing serious, and i'm probably one of the worst hypochondriacs you'll find here.
Also just to make you feel better my dad is a Vietnam Veteran and he has had many many "Spots" on him from the sun in Indochina, mainly Basal and Squamous Cell cancers. Only one time did he have melanoma and it was caught immediately, also it was "Insitu""SP?" which means that it grew horizontally and not vertically, which basically saved him.
Melanoma is an oddly shaped mole, it isn't perfectly assymetrical like most moles but is strangely shaped... kind of like what you would find in a babies diaper. Also you would notice it growing, and it would take longer than a few days..