פירוש על עבודה זרה 86:12

The ring that they made for R. Judah had a protruding seal, and it is prohibited to wear such a ring. One should not wear such a ring because it looks like an idol. But one can seal with such a ring because the image is sunken in, and therefore does not look so much like an image.
But if the ring’s image is sunken in, then it is permitted to wear it, but one may not sign with it, because such a sign would be protruding. In short, protruding images are prohibited, but sunken ones are not.