Lab Accident: What happened to my hand?

Hi everybody!! I spilled a certain chemical on my hand in the chemistry lab. I noticed that some half an hour later. There was a white stain on my thumb, which slowly grew darker, and finally became black. It does not hurt or itch, but i'm unable to wash it off(soap, vinegar nothing seems to work on it). I think it could be dilute NaOH or concentrated H2SO4, as we were working with these in the lab. Does anyone know what it is, and how I can get that off my hand?