Thanks Andreas for your suggestions.
What you see is only a test made on 50 euro cents
To illuminate the coin I used a Ring-Flash, but being very close to the subject the light practically hit the object only from the sides and not from the front.
Unfortunately, on the objects that I will have to photograph for the museum, I cannot use anything to make them less shiny
I will definitely try to use a light tent to diffuse the light and I will try, at this point, to use other lenses.
Thanks again