MacBug, I think you might be happier starting with another ring. You might be able to get a stone bezel set on top of your ring, but you might be able to start with another ring that already has a stone. These days, lasers can be used to engrave in stone, but the traditional intaglio carving can easily be distinguished from laser work in most cases. Being a smart gent, you might either spend some time with a Dremel or Foredom tool and some onyx scraps, or you might consider carving a wax blank to be cast. I don't know much about either, but that has never stopped me before.

There are plenty of online businesses offering rings like this. I believe they are expensive. http://www.familysealrings.com/Gemstone.html

However, if you are not married to a single idea or motif, you might spend six months or so observing the marketplace and consider buying an existing ring with a crest on it (NOT arms, but the head of a heraldic beast, which might actually belong to many different arms). There are over 500 items listed today on eBay under the description "intaglio ring" with subjects ranging from initials to kookaburras and a wide range of prices. Of course, not all are suitable for sealing, but many of them are.

We look forward to further developments.
Thanks
M'll