Since I Was About 15 Years Old (i'm 24 Now),

Since i was about 15 years old (i'm 24 now), i have had this little ball under the skin on the top of my hand. It started out very tiny and almost a blue color. Its now triple the size (maybe like a smaller sized seed) and is black. Its alarming since i'm told stuff like that is signs of skin cancer and me being a fair skinned redhead i am afraid of that. It doesn't hurt usually unless i accidently hit it off from something. When i asked my doctor about it she said maybe some sort of calcium deposit? I also was in a rush that day and did not tell her how long i've had it. I would really like to figure out what it is because it really bothers me there. Thank you