a ring with a diamond in the signature of the ringmaker?

Answer 1

I'm not sure what you are asking here. Do you have one, want one, or need to know if it's a good style?
If you want one, look at high end jewelers, like Tiffany & Co., or as above, Cartier.
If you have one and need to know who the maker is, take it to a local jeweler. They can tell you by looking at it, and they shouldn't charge for it.
If you are thinking of getting one, but you aren't sure if you should, that's up to you. If you like the ring, then buy it and wear it. That's a personal style, not something you should be asking strangers about.
Sorry I couldn't be more help.

Answer 2

Cartier?