The only problem that you could have is that when you use "Doctor" or "Doc", you may confuse people into thinking that you are in fact a doctor. This is especially true for your global audience. You can then be in trouble for "impersonating" or holding yourself out as a medical doctor."Doc" would be better than "Doctor" since "Doc" is a general nickname.Another way to avoid this potential problem is to make it clear on your website that you are not a medical doctor nor do you practice medicine. Specify that you only sell and engrave medical jewelry.