Ok, over in the "Heraldic Beasts?" thread, cajunscot posted a link to a primer from the American Heraldry Society that discusses, in part, the American heraldic traditions and history. I found this article, Guidelines for Heraldic Practice in the United States, on the site.

I am copying the following from that link:
4.3.2. Signet rings are still sometimes used to seal envelopes, particularly of personal correspondence, although modern mail processing equipment usually
ensures that the seal does not survive its trip through the postal system intact. Even if unused, however, signet rings can make an attractive and conservative
piece of jewelry. Signet rings have the arms, the crest and motto, or the crest alone engraved on a metal (usually 10 to 14 carat gold) or stone surface.
The engraving is done in reverse so that the design will appear correctly when pressed into hot wax. Many authorities state that signet rings are correctly
worn on the little finger of the left hand, but customs vary from one place to another.

4.3.3. Women who use signets sometimes have the arms engraved on a small medallion worn as a pendant rather than in ring form.
I read the primer, which I found helpful, but have not had time to go over everything in this article and the rest of the site, except for the article on registration.