What Causes Dark Skin Patch On Finger Joints?

Doctor, I am 32 years old and i have got thick dark patch skin at the side on my finger joints. It starts from small spot and pencil dot and spreads which make my finger skin ugly on both palm side and back. But I don't have any paid or illness, so please guide me what it actually may be. I fear it as any dreadful disease because it won't pain. Please help me.