Hi Diane
Good questions. If you have a good idea for a niche site and you are thinking of a URL to match the content you should make the URL something that is easy to remember for future direct type ins. Using diamondnecklacesauthentic.com is not an easy URL to remember for direct type in traffic unless you are Yoda
I would focus your site content around your three keywords diamond, necklaces, authentic so you can rank well in search engines for any combinations of those key words.