I left a stainless steel based height gauge (current affordable Asian DRO type) on my dark granite surface table for ~6 weeks. I now find that just where the stainless steel base contacted the granite, it's markedly lighter in colour. The area between the two legs of the base is the same colour as the bulk of the table, so the discoloration is confined to places in contact or very close to being in contact. (There doesn't appear to be any oil/grease treatment on the stainless steel surfaces, but would I detect any anyway?)
Do I just ignore the colour change - I can't feel any difference in surface texture, but then I've no idea how sensitive my delicate finger tips are!! (i.e. I'm not a builder with hands ravaged by cement etc.)
Cheers,